Jermaine McGlashan's stoppage time equaliser secured a point for the Robins against former Premier League side Portsmouth.
The visitor's took the lead inside five minutes as Agyemang caused the home defense problems and reacted quickest to poke Pompey into the lead.
McGlashan's first goal of the game came as he split the visiting defense and latched onto Terry Gornell's through ball before rounding the keeper and placing the ball into an empty net.
Portsmouth regained the lead before the break as the unmarked Johannes Ertl had time to pick his spot and fire past Scott Brown.
But the Robins finished the game the brightest and McGlashan's stoppage time goal secured a point for the home side, before Terry Gornell put the ball in the net from an offside position. The fans erupted but their cheers were muted as the linesman rose his flag in the corner of the pitch.
Scott Brown captained the Robins as Mark Yates made 3 changes to the side that started against Bury last Saturday.
Deadline-day signing David Noble started for Cheltenham following his debut appearance in the Johnstones Paint Trophy on Tuesday evening.
Jombatti started once again in place of the injured Braham-Barrett and Gillespie featured as Penn dropped to the bench, Yates sticking with the same formation utilised against Plymouth in the JPT.
Young shot-stopper Harvey Rivers retained his place on the bench whilst Connor Roberts is still absent, representing the Welsh-u21 international side.
The visitors made just the one change following their victory against Torquay as Patrick Agyemang started for Pompey.
The teams emerged from the tunnel to the sound of the Kaiser Chiefs, Ruby, blaring out over the tannoy muffled by the infamous 'Play up Pompey' chant from the away end.
The visiting fans had plenty to cheer about as Patrick Agyemang scored after just five minutes to give Portsmouth the lead.
Steve Elliott's slip enabled Agyemang to latch onto a weak Pompey thtough ball and the striker reacted quickest to slide the ball past an advancing Scott Brown.
Agyemang was causing problems a plenty for the Robins back line.
Matt Richards conceded a free kick whilst trying to prevent the big front man from advancing into the Cheltenham box and he missed a wonderful opportunity to double his tally for the day as he fired over whilst trying to regain his balance on the edge of the six-yard area.
The home side were struggling to retain possession, Pompey's defence were wise to good attacking wing play from Deering and McGlashan when the Robins defense eventually managed to find a way out of their own half.
The closest the Robins came in the opening 20 minutes was a David Noble ftree kick that flew over the crossbar into the Pompey fans in the Hazlewoods stand.
Neat interplay between Richards, Noble and Deering demonstrated the Robins wern't to be taken lightly.
The midfield trio fed Terry Gornell but the stikers poorly timed run quickly had the linesman flagging for offside.
The injured Ryan Inniss was replaced on the half hour by Russ Penn. Lowe filled the centre back position left vacant by Inniss. Jombatti moved to right back and Richards slotted into left back as Penn added strength to the midfield.
Penn's introduction lifted the crowd and the Robins were able to capitalise as Jermaine McGlashan brought the scores level.
For the first time in the game the Pompey defence were outwitted as McGlashan's diagonal run was spotted by Gornell. His quick thinking found the winger in space, advancing on goal.
McGlashan rounded the keeper and had the simple task of sliding the ball into the back of the net.
That goal gave the Robins confidence as they moved into the final 10 minutes of the half.
McGlashan won a corner shortly after and fired over from range before Gornell's weak shot calmly collected by Sullivan.
But Portsmouth did well to get back into the game whilst the Robins were on top.
They regained the lead to take into the half-time break through Johannes Ertle.
Jed Wallace's determinded run to the byline enabled hims to find Ertle unmarked at the backpost. The midfielder had time to pick his spot and fire his shot past Scott Brown to regain Pompey's lead.
Ricky Holme's will have wanted to do better as he should have made it 3-1 going into half time.
Agyemang's strength enabled him to round Lowe with Ease. The striker pulled the ball back to Holmes but his weak shot was comfortably blocked and cleared by the Robins defence.
Half Time: Cheltenham Town 1-2 Portsmouth
The second half inspired few opportunities on goal. Until the 90 minutes ticked into injury time.
Andy Barcham was involved in the majority of Portsmouth's attacks in the second half. His clever bicylce kick whistled over the bar before being denied by Scott Brown one-on-one shortly after.
Jed Wallace continued to look threatening but was unable to put the ball past Steve Elliott in an otherwise open goal and Scott Brown had to be wise to Holmes' curling corner as he palmed it over the crossbar to prevent a third goal.
Young winger Zack Kotwica replaced a tired looking Steven Gillespie and Deering made way for Harrison as Yates attempted to shake things up in attack.
The substitutions had a positive affect. Russ Penn's volley bounced just past the post and there were loud appeals from the crowd for a penalty as Sonny Bradley did well to deny McGlashan who had been unleashed in the box courtesy of a neat Noble through ball.
Noble, Penn and Richards were able to demonstrate how effective they could be as a midfield trio. Richards pushing forward from left back aided the midfield duo in commanding the game.
Clever crosses from the right wing twice found Kotwica who was unable to aim his headed efforts on target.
A trio of Pompey substitutions seemed to take a lifetime to complete and resulted in 5 minutes of added time.
In the first minute, McGlashan was on hand to equalise.
He capitalised on an error in the Portsmouth defence and rolled the ball past an uncertain Sullivan between the sticks and had the home crowd on the feet as the ball trickled over the line.
Gornell put the ball into the back of the net again shortly after and the Abbey Business Stadium erupted into the noise amid a goal mouth scramble.
The cheers were quickly muted. In the corner of the ground, fans slowly realised the linesman had his flag raised above his head for offside as the final whistle blew.
Full Time: Cheltenham Town 2-2 Portsmouth
Cheltenham Town FC (4-4-2): Brown (c); Elliott, Inniss (Penn '32), Jombatti; Deering (Harrison '66), Richards, Noble, McGlashan; Gornell, Gillespie (Kotwica '57).
Subs not used: Taylor, Vincent, Rivers (GK)
Portsmouth FC (4-4-1-1): Sullivan; Moutaouakil, Devera, Bradley, East; Wallace, Ertl (c), Ferry, Barcham (Cooper '78); Holmes (Bird '78); Agyemang (Padovani '78).
Subs not used: Smith (GK), Butler, Harris, N'Gala.
Referee: Mr. Brendan Malone
Attendance: 4,776 (1,560 visiting fans)
Star Man: David Noble