I had an intermittent problem at one time with my AC, in that it would run fine a little while and either cut out or go into FULL HEAT mode. This sucked in August. Once it "warmed up" a circuit board component would fail. Once cool it worked again.
My educated guess is that you should swap your PBU.
A rebuilt Push Button Unit for my '87 260E was under 175 with a fair warranty.
75% of the AC electronics are in that one box. Right there with the climate control buttons. Easy to replace with only a phillips screwdriver.