I have been paying more attention to mommies car and the thing I find in her's is that it tends to change over to defrost for a few minutes or seconds upon acceleration which would lead me to believe it is a vacuum issue. Once you back out of the throttle it takes a second but then shifts back to the normal footwell heat.
I'll have to check the vacuum supply and see how much the car is putting out. I would have thought that they have a reservoir for the climate control if it is managed like the diesels.
Will let you know if I find anything interesting.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique