Plenty of heat......
Replaced my monovalve insert this week. All the strange symptoms gone. Everthing works fine now. Took abt 20 minutes on my 1983 300SD. Make sure the coolant is not to hot as you will lose some which you'll have to replace.
I would recommend doing this first rather than letting the problem rent space in your head. 1 - Because at some time you will have to replace it anyway - and 2 because it's inexpensive and easier than trouble-shooting the climate control system.