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.

Does the volume of air coming out of the vents decrease? Or does it remain constant?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The air output only changes with blower speed, nothing out of the ordinary. The blower starts at about 1/2 to 3/4 speed which is probably related to the car interior temperature, may vary slightly after a couple of minutes as it approaches the temp setting and then increases to full output as if the temp was rotated down and clicked into the coldest setting at the end of the wheel.
All the controls act normally. The fan speed will slow if the temp wheel is rotated toward the heat setting.
I think the compressor may be shutting off at the control point where the fan would start slowing as the interior temperature nears the setpoint.