It's the relay...
I have the EXACT same problem with my 1995 C280. I ordered a new relay from partshop a few days ago. Actually, I can't wait to get home and put it in, if it's there. I think it cost me 40-something bucks. My wiper is IRRATIC! It will do just as yours will. I had it on intermittent all day (no it wasn't raining), just to see when it would go on. It went on twice today (between the 2 hours i was driving), both times when I was accelerating hard, probably coincidental. Anyway, I know that mine is bad because first of all this is a common problem and conclusion, and second of all, i located my wiper relay and turned on my intermittent wipers. The didn't turn on, but when i nudged the relay a little, it turned right no. It's a common problem. The relay for my car is under the hood behind the fuse panel under a black covering; I'm sure any other people with your call can you help you locate exactly where yours is. Good luck.
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!)