Rough idle, stalls

Make/model: 1998 Nissan Altima GXE
Mileage: 90,000
Engine size: 4 cyl

My car idle speed jumps from low to high, back to low then stalls out sometimes. I have had a tune up, new exhaust manifold and all hoses and sensors checked by nissan mechanics. So they say, But I really dout it. There is no code showing when they test the car. Also, when my car gets to speeds 45 or higher, the car coolant thermistat drops from half to almost cold and jumps up and down until I slow the vehicle down. I have no idea what to do.