[jeffcoslacker]

You've got a bad ground or crossed up wiring in the dash...I doubt the car is actually heating up that much (was that actually checked?), the guage is malfunctioning due to the problem in the electrics...

I'd start by pulling some fuses, like to the stereo, instrument lights, etc, and see what effect it has, etc...working your way through the affected systems until you see things calm down, then you have at least isolated the circuit causing the problem...