If temp runs OK at speed, this would appear to mean the radiator is working effectively enough to keep temperature at minimum (tstat tries to keep a minimum temp, right?) ... so, how can it be the radiator?
Have you changed the tstat? If not, give it a try ... what do you have to lose?
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