My HVAC problem was "simple", and finally solved. There is a plastic tray that collected condensate on the outside of the coils to keep drippage from getting on electronics. The hole in the tray clogged up, and it wasn't draining. A sensor would tell the compressor to turn off. Open the hole, replace the sensor, problem solved.
It's not easy to spot the hole in the tray, the unit is inside a closet, it's a water sourced heat pump.
The sensor, about the size of a dime, cost $170.