I am up here in Winnipeg Canada, where it is a cool 16 degrees Celcius. When it's really hot (25+) and very humid my car runs like a race car. But when it's cool like tonight it runs like crap. No vacuum leaks, smoke tested. Odd thing though when I pump the brakes very lightly in rapid succession the RPM's go up 50-100, weird huh? So here is what i have done to try and isolate the problem. I have taken of the Exhaust manifold heat shield tonight and the car is driving considerably good. I am the second owner of the car so I know that the Thermostat is the same one from years back. I own a 04, Alero. I think the thermostat is opening up a bit to soon. Anyways, if I am way of let me know. I appreciate all the help I can get. No codes have ever shown up at all. Car has 84,000 KM's. I know the engine coolant is stock because it's that brown crap that is hard to see in the oil. Would running less than 50/50 coolant be the culprit? Recent oil change with 5W-30, oil filter, gas filter, injectors and fuel rail cleaned and I also indexed the injectors. You would not believe all the metal filings I found in the fuel rail, must've gotten in there when they threaded them. Fuel pressure has been tested and verified to be within specified parameters. I took the car out when it was 25 and let er rip for a good 1/2 hour going about 160 KM, she purreed like a kitten. So that should eliminate the CAT.