2.2L stumbles after warmup

I have replaced just about every thing. cooling sensor, knock sensor ign. mod., coils, wires, plugs, computer, head gasket, yes valves were fine. i checked them when i replaced gasket. engine runs wonderfully, for about tem minutes. then stumbles like crazy. oh yes i put a o2 sensor on it also. :confused: I need some help. does anyone have an answer for my problem. besides scrapping the engine.:rolleyes: (mechpete)

check the catalytic converter

I have a '97 Cav with the same problem. Runs fine when cold, but stumbles from idle to around 1,200 RPM. Runs fine from there on. I've changed coils, plugs, plug wires, fuel filter, air filter, fuel pressure regulator, and injectors. No "check engine light". I have been told to check fuel pump pressure. Most likely, the pump is on its way south. $300 for the pump and a day to install.
Good luck.