Same car, same effect ... different circumstances.
I recently replaced the fan clutch and water pump on my 350 SDL ... it, too, had always run at 80-85C - always! ... after change it now runs there during low temps and/or limited load, but when load increases, creeps right up to 100C ... with AC on at 85 MPH it runs near 100C ... used to run 80-85 during same conditions.
Is the viscous clutch the issue? Am replacing thermostat when it gets here ... eliminating cheap and easy first ... only other thing is radiator, but what is the chance the radiator goes clogged in a binary sense ... don't they usually degrade a bit first as a warning?
The viscous clutch is a neat idea, but would a plastic deformable blade running unclutched work effectively, too?
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