96 Monte Carlo Running Rough

I just moved from Minnesota to Virginia. I have a 96 Monte with 3.1. I was driving one hot day with the AC on temp gauge was at about 3/4 car started running rough and died. After about 20 minutes the car started again. I took it to the shop and they said I had anti-freeze on the plugs and 02 sensors. They changed all that $676 later. I pick it up from the shop and 3 miles down the road the service engine soom light comes on. I take it back and the say the fault is low oil level. My oil level was fine. They reset it and off I go.
22 mile later the light is back on. The car runs fine if the temp gauge stays left of center. Once it goes right of center the car runs really rough when it goes to second gear from a stop. On the highway with cruise control it's fine. Anybody have any Ideas?

Is it possible the catalytic converter is plugging once the car heats up.

Is it an electrical problem because both rad fans and ac are running?

If the car cold natured and hates the heat of the south? LOL

The car has 187,000 and has be very well maintained.

Any help would be great thanks Kevin

First I would get my money back cause they did 100% of nothing to fix your problem. Then go to the dealer.

If you have anti freeze on the plugs....or anywhere else, sounds like a blown head gasket to me. Check compression on each cylinder to find the leak. Could even be a warped head if the engine overheated.

At 186,000 miles, it might be time to trade up. After all, this is a Chevy, not a Toyota.

Good luck.