loose steering at high speed

I own a 190 E 2.6 and the steering constantly needs adjusting when I'm driving over 120kph. Also, what should my r.p.m.s be at 120 k.p.h                       

 

You may need a new steering shock absorber. Had a simmilar problem on E420, wasn't too expensive to fix.

I Would Inspect the Tire Pressure first
Then Have a Look at the Shocks BOTH the Steering dShock and The Shocks on the car Also Have The Suspension Inspected for worn control arm bushings ...
And Last have The Alignment Checked.    

My guess - steering damper or some call it steering shock absorber.

Did you get the problem resolved?

Cheers.