It does sound like an overheat problem, the fact that it has run for 10yrs OK probably means that some component or other has maybe degraded slightly with age, which can happen with electronic components.
If the ambient temp is 120F, it could be alot hotter on the surface of a component.
If it is a critical piece of equipment it might be a good idea to have a programmed spare on hand.
BTW. you can get temperature indicating tape that can be stuck on various surfaces that register a max temp detected by colour.