L507809

I have a '97 Cavalier with a 2.2 motor. The problem started with a hesitation, which it still has. No check engine light. So far, I have replaced: plugs, plug wires, coils, fuel filter, air filter, fuel pressure regulator, throttle position sensor and idle air control as well as pulled and cleaned the EGR valve. Also spray cleaned the throttle body. Car runs fine when cold, but problem appears when at operating temp. Idle is slightly rough at about 600 - 650 RPM in drive. When throttle is depressed, tach needle drops to about 500 RPM, headlights dimand is very sluggish until around 1200 RPM, then runs fine from there on. Lights also return to bright. Today I replaced the idle air control. Car seemed fine when cold, but when warm actually is worse. Idle now floats from 600 - 700 RPM and surges at lower speeds. I read somewhere that the IAC will reset itself, but have just parked the car. Several area shops have stated that it could take several hours of diagnostic time at $75.00 per hour. It's getting to the point that the car is not worth any more $$$. Any help would be greatly appreciated.
Thanks