My '84 300D was slow to warm and then never made it to the 80C mark. I pulled the thermostat and found it literally in pieces. With a new one in place, the car warms quickly and the guage is steady at just over 80C. It produces noticeably more power at the higher temp, too, which is nice.

Also, while you're in there, it's not a bad time to change the short coolant hose beneath the thermostat housing.