Wall thermostat problem

[Scott Forman]

I have a problem in one of my wall thermostats. The one in the bedroom
that controls the propane furnace and chassis heat will work fine for
some period of time, but sooner or later (usually sometime overnight
while I am asleep), it quits shutting off and the heater runs
continuously. While I guess I could just consider it a sauna, I'd
really like to fix it instead. Any ideas what might cause this?

Scott Forman
86 PT38
Memphis