I had the EXACT same problem on my 1995 C280 awhile back. I think it was the right side (maybe the left side) of my speedometer (lights) would not turn on. Well I would start the car, drive for a little while, then they would turn on. Or I could smack the dash, then they would also go on. Anyway, this happened probably 10 times, but has not occurred again in months... Seems to have gone away. Maybe a loose bulb or something?
I have a loose connection in my left side parking lamp on the rear (the inside one of the two), it sometimes fails to work for minutes on end (I know because of light indicator in dash, but will work again and not come back for weeks or a month)... Strange....
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!)