Hello.

Driving at those speeds at that RPM is not good for the engine, especially it is driven a long time like this. If I recall, doesn't those cars "redline" at 5500 RPMS? Is it a V6, or does it have the V8 option?

Perhaps you are in need of a transmission filter change. Transmissions are complex, so I would recommend taking it to a transmission repair shop that you can trust.

Also, this is a Mercedes site, rather than Mercury/Ford, but hopefully someone will have some info about this.