breaking up///misfire

Iam having a problem with my motor.It is breaking up(popping through the exhaust) around 5,000 rpm in second gear .if I try punching it in 3rd gear breaks up about 4,000rpm.I switched coils still doing the same.a breakup or misfire what would cause it.the car ran great two weeks ago at the track.The only thing I did since then was get fuel from a local guy out of a drum (Sunoco 110) could it be bad gas,or an electrical or vacuum problem.
It has been awhile since I changed my cap and rotor.I will be getting a new one this week.


John

Though an exhaust backfire usually points to a rich condition, but if all you did was put the gas in, you may just have bad gas.

It is not a backfire .It is more of a miss,wont rpm past the mis and it is more noticable when it is under load.It is not doing it in first gear
Thanks for your input

Also Iam running a 6al with the billet msd dist.Could it be the rev limiter chip or a bad ground?

My car did this alot,when we cruised around town slowly and the enginebay got really hot.When I hit it it just popped.I think the gas is boiling and or the feed to the bowls is not enough.I have upgrade to 3/8" lines and rerouted them and have tried one time at a cruising to hit it:It screamed.