GMC P/U AC Problem

I have a 2005 GMC Sierra 1/2 ton, 4x4, crewcab with 62,000 miles. During the summer the AC started blowing very hot air, engine temp air. I took it to my mechanic whom I’ve been using for 15+ years. The compressor was diagnosed as failing, which I could validate by reading pressure readings. New compressor $900 parts & labor. Two days later same problem. New diagnosis. Servo motor directing airflow failed allowing engine compartment air into AC ducts. New servo motor $380 parts & labor. Problem recurred but now find out the hot air is coming only from the two ducts pointed at the driver. The two passenger ducts are putting out cold air. My mechanics are stumped. It recurs at odd times–cannot figure out what triggers it. Somedays I can drive for hours with no problems. Other days it happens after driving 15 minutes. When it does recur I’m never near my mechanic’s garage or it’s when they’re closed.