surging while driving

hi, my brother in law is having a few issues with his 04 2.2l cavalier. he has slight surging at idle between around 700-1000RPM but the biggest issue is at around 1500RPM. while in park if you blip the throttle, on the way back doen the rev range it sticks for a second at 1500rpm and then drops back to it idle. however while driving if you let off the gas (at anyspeed so long as the rpm's are above 1500) the needle drops to around 1500 then starts wildley surging between. wouldnt be so bad but he has modified his exaust so its loud as hell and this engine surge is very noticable by everyone within a half mile radius :rofl:

hes gone through all the simple things, changed his plug, replaced his fuel filter as he thought it may be blocked, im thinking next step is either the throttle position sensor, IAC or O2 sensor but they would be just my guesses, anyone have any better ideas of what to do?

okay cleaned the IAC (very carboned up) wasnt that cos the problem is still happening, gues its gonne be a trip to the junk yard for a mass air flow sensor, o2 sensor and TP sensor cos i know hes not gonna wanna pay autozone prices especially given that i dont know which if any is causing the problem.