Wanted to follow-up with my results from replacing parts.
My 350 SDL's problem was covered early in this thread ... it simply started getting hotter (never overheated, but would rather easily run to the point that it would kick on the electric fans ... 105C, I think?) than it every had, and, this corresponded exactly with replacement of the viscous clutch and coolant pump - I had replaced the viscous clutch to cure a wobble which was part viscous clutch and part coolant pump.
I replaced the tstat and put on another viscous clutch - not scientific since I should only replace one at a time, but I was not gonna tear all the stuff apart and drain it twice!
Result is that it runs just like before it started getting hot ... it appears to come up to temp faster, and then stay at 82-85C in most driving. Today it was 98F and I was able to make it move, but even in stop and go with AC running, with almost no airflow over the nose, I was unable to get it to 100C ... just would not touch it. As I started moving even a bit, it immediately would run back down to 82-85C.
I wish had more time and money to address the issue more completely - an item at a time - would be nice to KNOW. I suspect the problem was the tstat ... I don't think the viscous clutch should be a factor with ram air at 50 mph, regardless of the outside air temp. I boiled it to watch it open and close, which it did, but I don't think this is a valid test on this tstat.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg