My 300D sometimes behaves the same way, and its thermostat is only a month old. I have heard of these thermostats being defective right out of the box, but I'm still not quite assertive enough to demand that my mechanic replace it again. I will be interested to hear the outcome of your problem.
[Edited by Ben300SD on 05-30-2001 at 02:12 AM]
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K