84 300 CD..When driving, with the temp control on max, (and any of the air position buttons pushed)the fan blows plenty of cool air. If I stop and idle, then suddenly there's loads of hot temp heat from whatever position I choose. Get back on the road and it cools back down... While I wait for my service CD, is there anything I can look at immediately (there's frost on the ground now when I leave for work in the AM)

Remove the mono valve insert and inspect the diaphram for tears. I suspect that you will find one. Replacement is the repair. A kit is avilable with a new insert. No need to replace the complete valve.
Good Luck.

Corky: What is the mono valve and where is it located? Hate to sound clueless but in this case
No CD = no clue

Mike:
The mono valve is located on the fire wall just to the left of the battery. It has a two pin connector on the top with a cover that has four screws holding it down. It has coolant lines (three) attached to the lower part of it (plastic) Removal of the four screws on the top plate is all that is required to remove the insert and inspect.
Good Luck.

Mike,
My 83/300CD had the same problem last winter and what Corky explained was the fix for my car.

Good Luck