Yeah, I am thinking something to do with the defrost too. I have been out on two calls tripping on high head lately that the residential techs were having problems with and found both to be problems with defrost. One had the wrong defrost termination switch on it that was causing it to stay in defrost too long and the other was a problem with position of the switch on the tube.
Originally posted by HVAC Pro
Make sure your defrost thermostat is making good contact with the tube. I've seen them get turned a little causing the groove in the switch to not fit well, which can prolong defrost allowing pressures to trip the switch. Compounded by warmer outdoor ambients.