The temperature in my 300SD behaved almost exactly the way that you're describing until I did a citrus flush. Now it's always between 70-80C even under severe conditions (stop and go traffic + warm weather). I bet you've got the same problem I had.
I attributed my problems to *internal* obstructions in the radiator reducing the coolant flow, mostly scaly white deposits from hard water and corrosion... has your 617 ever had green coolant inflicted upon it? I could see deposits anywhere I looked in the cooling system, including on the insides of hoses and in the thermostat housing. They were gone after I did the flush.
The citrus flush is a bit time consuming, but not difficult (unless you snap a bolt in the thermostat housing while opening it up like I did
). First you remove the thermostat and coolant, then refill the system with a quart of shout and water and drive around the block with the defroster blasting. They you flush the system with water a few times until it stops coming out soapy. When doing this it is essential to open the drain plug on the engine block to completely drain the fluid on each rinse or you'll never get all the soap out. Then fill it with a citric acid solution, drive hard for an hour or so, and rinse it out a few times.
The difference was astonishing. After the soap, every rinse was coming out a rusty brown color, but by the last rinse after the acid, the water was coming out as clear as it went in. My engine would frequently touch 100 before, and never drop below 85. Now it consistently runs between 70-85C.