Just wondering if you have ohmed out your interior as well as exterior temp sensors for correct readings? I would also look to see if the interior sensor bottom air flow tube has come off or is deteriorated. I had a similar problem on my '80 300TD and found that the temp wheel needed calibration and was pretty easy to do yourself to see if it was set right.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan