Ben, I've been chasing a similar problem on my 1989 for 4-5 months now, without any sucess. The problem is not exactly similar to yours but very close. Mine turns cold when I need hot and turns hot when I need cold (in the summer), only when the car idles. It reverts back to proper temperature by simply accelerating or having the engine turn at 1,200RPM or more.
So far, I have spent close to $800. on unsuccessfull diagnostic, replaced the aux. water pump, mono-valve, checked the circuit board on the climate control panel, checked the temp. sending pump.
My local Mercedes dealer says he has never seen this problem before........
Good luck
jackD