History:

Sporadic warm running with A/C.

It will run perfect sometimes, other times I look at my guage and it is quickly approaching the red. Off with the AC and turn
the heat on, temp goes down. Again, this does not happen all the time.

I am leaning toward the fan relay, but would it work sometimes and sometimes not?

Result: Replaced The TStat, problem solved so far, that was 5 days ago, temp guage dead center now.

Thanks for the help, Art