Mine has been doing the same thing. I noticed that the accelerator pedal stuck down to the floor once while doing this, so I am going to guess that it is the linkage and/or ETA freezing up. Once it warms up a small bit, it is fine.

__________________
1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out