Listen to M.B. DOC on this one. Your speed sensors (conductor plate) need to be replaced. I just did mine a few weeks ago.

The part has gone through several revisions. The part number M.B. DOC has is the latest (since last month) at a reduced list price (a good surprise).

I will even go out on the limb and say that your car actually stuck on 3rd gear when it happens. Do you feel like it is hard to get it going from a stop position? 1st or 2nd gear will not get you that (sluggish) feeling. In 3rd gear, 40 MPH will run about 3000 RPM.