Temp Gauge Working Intermittently - Suggestions?

If the gauge works fine until the water gets hot I would suspect temperature related. Since the sender is the hottest piece of the circuit, a new sender could be my first move. I dont always start by throwing parts at a problem but... a small part that doesn't cost a lot and is easy enough to replace just might save me a whole lot of trouble. Some instrument panels are more difficult to R&R than others.
If you do end up pulling the panel and find a loose nut behind the gauge, same 10-32 hex nuts and internal toothed washers as found on the AMP gauge will work on all of them. Just dont over tighten. If the stud should turn it will break the fiber board it is sweged into.