If you havent flushed it yet be a good time to do so.

Id pull both radiator hoses and the themostat, stuff a hose down the thermostat hole and let it go. If your getting good flow, stuff the hose with a rag around it into the lower radiator hose and see if you still get good flow.

Assuming your flow is good, try snugging things back together without the thermostat and see if your getting good flow with your radiator cap off.

So far from what you have done, its sure pointing to a bad thermostat.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."