Well maintained car with 156k. Problem began yesterday. I tested the system today at 80 degrees outside temperature driving on semi-rural Pennsylvania roads at 35-40 mph with a few stop signs. Everything seemed normal on startup - fans, control buttons, temp set, etc. all act normally and the air is good and cold. I didn't measure the output with a thermometer, but a hand against the vent became uncomfortable after a few minutes.
As the engine approached normal operating temperature, the a/c blower which had been operating at about 3/4 top speed, with a soft click, increased to top speed and the temperature at the vent very slowly began to get less cold. I say less cold rather than warm because I never felt that the system had switched to heating. I measured the final output temperature after a 12-15 minute drive at 88 degrees.
Restarting the car after a half hour of cool down has no effect.

I replaced the mono valve a second time last fall when the heater would rise to the set temperature and then quit until the ignition was turned off and on again. I checked the mono valve for tears again this time but I didn't think to check the solenoid operation.
I have ruled out low R12 or bad compressor because the problem started suddenly and everything works fine on cold startup.

I usually do most general maintenance myself (oil, brakes, belts, body & fender etc.) and I am hoping for a solution before I start wasting time and money on useless experiments.

Thanks in advance for any help.