I had the same problem with my '88 260E last year. In hot weather or stop and go traffic, the temp would start climbing into the 90's and make me nervous. I replaced the original Behr stat with a Wahler (don't remember the temp but it was the same as the original) and the gauge now holds steady.

I also changed the coolant same time as the stat and found that the old stuff was way too concentrated. It pegged my tester at -60. That also could have been what was causing my cooling problem, but overall I'm happy with the Wahler thermostat and have always used them in my Volvo's too.