BB, look here (on a 126)!:

http://mbdieseldiy.tripod.com/monovalve.htm

Jazz,
Your symptoms are not the usual failure mode for a malfunctioning valve piston, the $30 part. That failure default mode is continuous heat, more at idle less at speed!I have heard of valve bodies being replaced, but it a pretty simple and sturdy part so if replacement is required a used part will be the most economical and a part that has been looked over and is up to spec should last forever. I guess your valve body itself could be damaged, corroded, plugged etc.
I have not heard of a coil failure but you can get that part used for sure and possibly new (the coil may only come as part of a new valve body?) Could be a control problem from a faulty ACC?

There are also three coolant hoses ( plus! two smaller hoses going to the windsheild defroster radiator) and their respective connections that may be the root of your problem. Once you see where the monovalve is located and the connections and how they need to be made, you will understand why a pro will not do the job for less, its a pain that will test your patience and resolve working in that space.

If you dive into there you might want to check out the auxilliary circulation pump that will be eazier to access with the monovalve assembly removed! Its just to the right (facing rearwards) and directly under the ECU! Maybe easier to take both out together? Good Luck!

This is another perspective,

126 Windshield Defroster Heat Exchanger ?