I've seen this same problem on a buddy's 97 C230, which is the same driveline. He had a CE light on, though. Weird that you don't.
He needed a new MAS, and problem solved. MAS's are problematic on most new cars, as stringent emission controls have somehow mandated a new design that seems more fragile.
One thing though, is that the MAS (in Canada anyway) is covered on the 8-year 130,000 kilometer (about 80K-miles) emissions warranty. I think it's the same in the US.
1998 C230 "Black Betty"