I too, have witnessed the same effects that you have. I have had my 84 300D since practically new and I noticed the temperature gauge was solid at 80 all the time, winter or summer, AC or no AC. After about 12 years or so, I began to notice the gauge fluctuating more than usual with readings on a hot day and with AC and going up a hill, I could hit 115. The point is now my front crankshaft seal needs to be replaced and I am wondering if I should do the water pump, hoses, belts, and thermostat at the same time. I think I will for piece of mind, but I always wondered about the impeller degrading over time. I will let you know what I find out.
'06 E320 CDI 51K miles
'02 VW Jetta GLS TDi 120k miles
'99 Ford E350 7.3L Powerstroke 200k miles -sold
'84 Euro 300D 5 spd manual 317k miles - sold