WEll - I've been doing my own digging and pulled apart my OVP - in the process I probably ruined it but it is working. I resoldered some of the connections on the back because I wasn't getting voltage to the "idle control valve" at the front side of the fuel distributor. (To check and see if - going on memory here - the OVP is sending out correct information to the "ICV", measure the voltage from the blue w/ red stripe wire - this should read the same as your battery voltage.) I was only seeing around 0.156V here so I took apart the OVP.
To my surprise, it hasn't stalled since - it still isn't running right but a whole lot better than before. The date on my OVP was Feb. 11, 1988 - so 13 years have definately taken their toll.
I will let everyone know what other changes I make and see if they help my poor running engine.
'85 190D 2.2 5-speed
'87 190D 2.5 (non-turbo)
'89 300TE station wagon