I'm not an expert, but I had a similar problem with (2) 300e's and (3) 190E's that I've owned. The way you described it, sounds like the climate control module is failing. This is the push button control unit and it has a microprocessor inside. The processor (computer) nearly always fails over time.

It's not user repairable. But you can find good used ones from Potomac German Auto in Maryland or Silver Star Recycling. Any large bone yard with Mercedes parts will have one or can locate it for you.

Best Regards,
William