Update: the A/C low-pressure port reading was accurate, verified with another gauge. It is back to normal pressure.
Finally got some time to take the IP apart enough to check both temperature actuators. Found the left (driver's) actuator not cycling. Removed both actuators and swapped them, to troubleshoot. Problem stayed with the original driver's actuator, now on the right (passenger's side). Took it apart to look for the broken gear mentioned by brcidd and adboll (another thread), but all gears were OK. Put it back together, and it worked. Temperature control seemed normal for both driver and passenger. If the actuator quits again, I will have a good idea what to replace.
Thanks for the suggestions.