Problems with car after exaust

[fordblue5oh]

Well guys I have a 2002 GT 5sp with 12xxx miles on it and i picked up half an american thunder 40 series catback system(minus tails)for $50 and i just installed it yesterday. Well it sounds pretty good even with the cats but now at idle i notice misfires just like soft "pop glug glug pop pop" this dosent seem normal to me and also now when i push in the clutch sometimes the idle drops down to like 350rpm and sounds like it is going to die. Anybody else ever hear of this? My car is still under warranty for like 3 more months and it would be nice to have some idea of what it might be before i take it in and they try to tell me "its normal" or change some part that has nothing to do with it and say its fixed(my ford dealer sucks its wham bam thank you mam now leave so we can run another 200 cars through today). Any ideas would be apprecated.