The best way to test t/stat is to remove it & put it into pan of boiling water. On all MB t/stats there is a marking that states the opening temp. Make sure that it opens completely. If this checks out then you have another problem.
A new t/stat with every coolant flush is $$$ well spent. Running w/o t/stat will allow engine to run too hot in summer weather & car won't perform as well over the life of the engine.