I have the same exact problem on my 85 300D (check out replys to my posting "heat will not come on"...and I'll check here also).

I'm thinking there may be something wrong with the auxiliary pump... maybe it is supposed to pump water all the time but for some reason it gets stuck shut when the engine is running...I'll check on that next weekend, and I'll let you know. How did you check to see that the monovalve works fine? (aionascu@yahoo.com)