If you are sure the trinary switch is opening when it shouldn't, like the thermostat is giving problems. Then you need to check the pressure on the system. You didn't say if it is a r-12 or -134a system. If it is/was a r-12 system and someone has retofitted to a replacement gas. That's a 'can of worms'. The experts say the pressure of a r-134a is much more critical and unforgiving than r-12. Did the guy who gassed the system really know his stuff?

Six volt guy living in a twelve volt world