Mostly during the winter months, my temp gauge (and occasionally tachometer) usually stays in the resting position for a few minutes or until I whack the dashboard. The needle sticks to the resting knob... something to do with the paint MB used.
Does your gauge work on and off? Or does it not work when first starting the car... and then kicks in again?
If your problem is like mine, I'd be pretty sure about the sticking needle.
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!)