98 Cavalier. Crazy Temp Gague / Hesitating

My wife has a 1998 Cavalier that she abused for several years before I met her. Basically no regular maintanance. I have had to fix several problems. Many I think were caused by lack of general care. Oil etc.
Anyway the major problem is this.
When she drives the car it will warm up a bit and then the temp gague goes crazy. Jumping from the Cold to the Mid way point. It never seems to go above medium but it jumps very rapidly between the cold and the medium. When this happens the car is -hesitating- and -bogging- down. It starts running like crap.
So my guess is this is some kind of temp sensor problem that is really messing with the computer. Any Cavalier owners experience this problem. Any other ideas as to what the problem might be?
2.2L
What I have tried to fix: The valve cover gaskett plugs and wires have all been changed. I also had the crank sensor replaced and the knock sensor? Still happens.
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as dumb as this is gonna sound, have you tried a thermostat?
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