I had the same problem with my tachometer on 1995 C280. Listen to the other's advice, it will probably cure it. Mine stuck on 0 for a few months. Every time I started teh car, I would whack the dashboard and the tach would come up. Leading me the obvious conclusion that it was sticking.
To make a short story long, it stopped sticking randomly and has worked EVERY time perfectly for months now... Knock on (Zebrano) wood.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)