[cellsafemode]

I'm 100% convinced that the thermostat 'goes bad' because the coolant level is allowed to drop and it becomes exposed to air.

this allows a film of dexcool to dry on the parts that need to smoothly slide and move. this causes the tstat to become stuck open.

this air exposure can happen from low coolant periods or parking on steep hills.

you can remove the tstat and clean off all the dexcool residue and it should work fine again.