Hello, I have an 83 300 SD with a temperature gage that seems to be malfunctioning. The gage will jump from 80C to 85C suddenly. Sometimes it will drop from 100C to 85 C suddenly. This seems to happen more if the A/C is switched on and off.

Has anyone else had anything like this? Shouldn't the temp gage make smooth transiitions?
I've checked the instr. panel for loose screws. No ground wires appear to be loose. Sensor was replaced 4-5 years ago with an aftermarket replacement.

Is this a grounding problem?