DMAX, even if you were in 3rd on the highway, your temps should have still been a bit higher, HOWEVER, the thing that makes if hard for all of us to follow is if and when the DME has the thermostat heater activated??
I am guessing maybe INPA may be able to monitor this, BUT I am now trying to figure out of and when you can tell EARLY on if the stat is starting to soft fail?
Can you condemn the stat when the idle temps drop as low as 88C?? 86C?
Maybe more data points will help us determine when it is time to toss a new stat in the car. The unfortunate parts is these stats are not cheap and there are not many options that will give economic relief. Did find a suitable Stant stat for the M5 with the inner seal for under $20, when the inner seal from the dealer at discount is over $40 and the stat was about $50 as I recall?
The one thing I can say is you only seem to find the states have failed during the colder months and many times you end up freezing you backside off trying to swap a stat in the middle of Jan.
Ask me know I know!