I could very well be wrong on this one... but...

Are you talking about the hose that runs from the coolant reservoir (from the top of it) and over the radiator?
Have you tried tightening the hose where it meets the reservoir? It sounds as if air is getting into the system, therefore the bubbles and the leak.
Have you done this? I would try tightening the hose, I think it can be done with a screwdriver.

Any other ideas?

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
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!)