Hey Dave, sounds a little like my 1990 190E 2.6. My a/c used to work perfectly now who knows what you'll get when you push a button on the climate control system. I can usually get heat any time I want but the a/c is a little tempermental. I have found that the a/c comes on and blows cold air out the center ducts but the side ducts will sometime blow hot air with the a/c on. If I switch it off for a few seconds and turn it back on it works as it is supposed to. I know my aspirator (that sucks air into the temperture sensor next to the sun roof switch) is not working. That may be the cause. But you know how it goes I will fix it when I have the time. Hope it helps. Good luck.
Jerry M. Wallace,