2007 Odyssey A/C works intermittently

My Odyssey's a/c stopped working last week, when the temperatures were pushing into the 90's (I subsequently tested it in colder weather, and it seems to work as advertised, though not as cold as I would expect). My initial thought was that it was low on R-134a, but it was still in the lower end of acceptable. I did top off the system, but that did not help the problem--it blows moderately cold for a couple minutes, then goes to ambient temperature (warm to hot). I've looked up troubleshooting for the transistor, but none of the symptoms match up (i.e., fan control works well), and all fuses are intact. I can spin the compressor with my hand when the engine is not running, and the clutch engages/disengages when the a/c is cycled. The low-pressure line is very cold when running, but the high pressure line is only moderately warm.
Any thoughts or suggestions for further troubleshooting? Thank you.