While driving home tonight I noticed my temp gauge was going down to 100 when I would come to a stop or slow down. Wasn't overheating but would go back to normal when I would give it gas. Makes me believe that overheating is next, what could cause this? I have a 87 cherokee pioneer 4.0 renix. Still has closed system.

 

Thermostat sticking open?

 

I'm thinking a simple loose wire. The temp gauge get's its reading from a separate sensor on the back right (Facing the engine from the front of the vehicle) of the engine, I had a similar issue (Mine didn't work at all) and the wire just popped out. It's a single wire with a plastic clip on the end.
When it;s disconnected the gauge reads minimum.

 

I'll check that out, Thanks.