Invested some time in the car this evening (and made a HUGE mess on the warehouse floor, but that's another story).

The thermostat housing is clear and clean. Burp port is free and the little ball is freely rattling. I went ahead and boiled out the housing in citric acid just to make sure it's spotless.

Moving on, I pulled the radiator and gave it a good flush. Good flow and nothing but water and coolant came out of it. No rust, no corrosion, no flakes, no calcium. Fins are straight and the A/C condenser is clean.

Starting to run out of options, I decided to test the thermostat by throwing it in a pot of boiling water. Before I tossed it in the pot, I attached a thermocouple so I could monitor temperature. I think we have a culprit... I measured 97˚C on the thermostat body, and it was only open ~1mm. I left it in the pot at full rolling boil for ~10 mins, the temp on the body was 100˚C and still no further open. We're talking a gap barely big enough to get a fingernail under. I don't think that's right for an 84˚ thermostat...

Ordering a new thermostat and trying again. Shame, it was a Wahler too.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]