Sounds like you are down to:
1. Bad radiator (corrosion may not be evident) as most expensive I would consider it last
2. Bad viscous clutch - also a very expensive item if someone else does the work ... can trouble shoot by locking it up by bolting the fan blade to the assembly, but, if bad, it will cause overheating at slow speeds
3. Thermostat - I know you said you put in a new one, but if you have done searches on tstats you will find they are very unpredictable - very cheap to replace, especially if you are doing it yourself ... try another one from another manufacturer and source
Me? I would start with tstat and don't change anything else. If that doesn't do it, then lock up the vclutch on the far and see what happens. Radiator is last ... and, expensive ... try getting a used one and giving it a try ... R and R is not all that hard.
If these don't do it, it has to be either a pressure issue (sounds like you believe coolant system is absolutely tight and since it isn't boiling over, I don't think that is it either); or, a head gasket problem.
Try the tstat - a lot of folks have been very surprised to find their new tstats don't work.
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