I had a similar problem in my 500SL and traced it to--I thought--the black Climate Control Unit which mounts behind the glove box. Replaced that and the problem quit.
Awhile back though my voltage regulator started acting up and I replaced it. Wondering if the CCU box...which MB now wants over $1,500.00 for...might have been affected by the voltage regulator, I reinstalled the original CCU box and it works like a charm.
So, bottom line to this is I think the voltage regulator was causing my problem. Perhaps yours could be so simple.
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000