Just a thought ---
Are any of the indicator/warning lamps on? I think when something in the ABS system has failed, the revs are limited to 4500 RPMs. Pushing past 4500 RPMs and it feels like a governor kicks in and keeps the car from passing 4500 RPMs no matter how much gas you give it.
In my E420 it turned out to be a blown fuse in the black box with 4 allen screws (on the passenger side by the firewall). This had the 4500 RPM limitation you're talking about, and the cruise control didn't work, and the ASR dash light came on.
I suggest checking those fuses; you'd hate to have the dealer tell you is was just a fuse and charge you 1 hr diagnostic time! That's what my benovolent dealer did!