03 cav base 2.2 ecotec.
It all started because my care was getting hot, I noticed that the fan no longer came on. It than began using a lot of coolant (but I watched it closely) as long as I didn't turn on my ac the temperature level stayed at 195, I alway made sure the coolant level was good. I was driving home and noticed that my car temp was at 220 so i pulled over let it cool. The engine light just flashed at this point but never stayed on. Eventually I read that that means it was like in a safe mode, long enough for me to get to a garage.

Eventually the engine light came on and i Got code 0400 multiple misfire, possible cat conv failure. I replaced the plugs, ran fine for 3 weeks
after replacing the plugs for the second time, making sure that the gap is correct because my car suddenly started behaving like it had no power and and with the engine running it is very rough. It is now throwing code 0300, more multiple misfires. I pulled the plugs again, put them back in and made it 15 miles down the road before it started misbehaving again. just as soon as the car cools enough I can go a few miles down the road then it will begin to act as if it has no power and running very rough. Please tell me if the cat will cause this or should i begin looking for another reason. I have ordered a cat.

Maybe a head gasket? How's the oil look?

Not sure if it's a similar problem, but just in case it helps...

My 2.4L would run fine when it was cold (~75 degrees) but then run rough and stall after it warmed up. It turned out that the temperature sensor (not thermostat) wasn't working properly and therefore sending bad (too cold) data to the computer, which then caused the computer to feed the engine an overly rich fuel mixture (like a "choke" for you old-timers like me).

IS the coolant level dropping over time. Sounds like water heats and builds up pressure then seeps into the combustion chamber causing the misfire. What did the plugs look like when you pulled them? (black, brown, white?)