Inspect and test all the following listed below: Oxygen sensor.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Manifold absolute pressure sensor.
Fuel pressure regulator leaking or defective fuel pump.
False air leakage.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.
Note: If it doesn't apply disregard
June, 20, 2010 AT 5:10 PM
Just fixed a similar problem with an 02 Camaro V6. Had bad cat converter (sounded like shaking a pebble in an empty can when cold, very noticeable).
Bad cat caused engine to run hotter than normal, ignition coil packs located on top of valve covers and hot EXHAUST MANIFOLD, finally solved problem when I replaced the ignition coils (found them cheap on eBay). Trouble was worst with engine fully warm,
A/C on, passengers in car, engine turning around 1500 rpm in higher gears ( max load, max ignition advance).
June, 20, 2010 AT 5:15 PM
Have a good independent shop plug in a SNAP-ON scanner to your diagnostic port to monitor the engine while its running, only takes a few minutes. If you see misfires on certain cylinders, focus on the coils/plugs/wires for those cylinders if your cat conv is ok. You probably have the single coil/wire/plug setup for each cylinder, the scanner should tell you.
Might even be a lose spark plug wire, make sure that they are all down TIGHT.