Backpressure problem????

I just put 3 inch catback exhaust on my gsx and replaced the catalytic converter witha test pipe also. I have been noticing a problem whne i punch it in second and third geers at around 4-5 thousand rpm. The car seems to hesitate like stall briefly and then continue normally after that. It jerks my head really bad and is a definate problem. My friend has a girlfriend with a brother that races gsx's pretty hardcore for some Archer racing and he said that its something with the back pressure to any advice on fixing this would be helpful.

what year is your car? what other mods have you done?

97 gsx with intake and filter, plugs and wires, 1g bov with uicp, boost controller at 16 psi and gauge, the piping and catalytic converter pipe, and my phat system and white 18" wheels. :) But i had all the mods done before the exhaust piping and so it must be something with that. Otherwise would it maybe have something to do with the plug wires? Because i put those on around the same time. Any help is appreciated.

so nobody has a clue what that could be??

It's not backpressure. My guess is that your new exhaust increased your boost, and it's on the verge of fuel cut. Do you notice it only happens under load? Sounds like too must boost, a boost leak, or spark plugs. I'm guessing the first since it was caused by exhaust. (it's common for aftermarket exhaust to increase boost) Turn it down and see if it does the same thing. Test for boost leaks... make sure your boost controller and wastegate are leak free.

Almost sounds like spark blowout...maybe gap your plugs a bit closer or swap them for a colder plug.
As Joe said, it has nothing to do with backpressure...I'm going along the sames lines as his guess...the opened exhaust allowed you to boost a bit more, this extra boost is causing spark blowout...either turn down the boost as he recommended or regap/swap the plugs.