had some weird engine temp fluctuations some time on the truck in my sig (60k miles). put in a new tstast and it is rock solid now.
our old IH hydro 70 w/D-312(bought it new is the early 70's) has the original tstat and it is going good after 40 years. though it is a low hour tractor 2,000 hours... it still has bits of red paint on the fan belt!