I had that intermittent problem for a few weeks on my 400E. It eventually did not blow air at all. It turned out to be the blower regulator. The good news is, if yours turns out to be the regulator, it looks like it would be easier to get to than on my 400E.

http://www.peachparts.com/shopforum/showthread.php?t=71166&highlight=420sel+blower+regulator


http://catalog.peachparts.com/ShopByVehicle.epc?q=1990-Mercedes--Benz-4--0sel-Climate--Control&yearid=1990%40%401990&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6266%3AMBC|1533%3AED|10000055%40%40420SEL&catid=242213%40%40Climate+Control&subcatid=242297@@Blower+Regulator&mode=PA

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles