1997 Pontiac Sunfire Several Problems

Engine Mechanical problem
1997 Pontiac Sunfire 4 cyl Automatic

Last month my car was running hot and I took it to the shop were they fixed it. I am a complete idiot when it comes to cars so I dont know what they fixed exactly but they said it wouldnt run hot again. Then recently it was running hot again. When I drive it goes back down to the middle but once im sitting still it goes to hot. And now the engine light has come on and it shakes when I start it up and when I sit still. It jerks like it will cut off once I pull off and when I start to brake. The last car shop I went to they said that I need to get something flushed and then they tried to put in new spark plugs but one of the cylinders wouldnt come off and to do that they would need to take the gasket out and that is $500..I am confused and lost as in what is wrong and what to do. Somebody who knows what they are talking about please help me?