My temp gauge disappears after the car has been running for awhile? What is causing this?

I purchased my Celica about a month ago and already had to put a different engine in it because it threw a rod through the engine block. Prior to the incident I noticed that the temp gauge dial disappears after the car has been running for awhile and it continues to happen.

I took the car in yesterday because the speedometer and odometer stopped working as well. While it was there they found a loose clamp and coolant had been leaking out. I have. To. Assume the car probably overheated some. Since the clamp has been replaced, the speedometer and odometer are working fine, but the temp gauge still disappears after it has been running.

I also have an issue where the odometer, fuel, and temp gauges are hard to see (but I have to assume that is only a burnt out bulb). They told me I need a whole new instrument cluster, but I'm concerned there may be an issue with the termostat or a sensor in the coolant system. Is this valid reasoning? What else would cause the temp gauge dial to disappear when everything else on the instrument cluster is working fine?