It could also be that the overflow tank cap has gone bad or has debris in the gasket area. Try a new cap first. Also check all hose connections. You may have a loose hose or a cracked hose. Also check the fluid level sensor. It may have developed a leak and needs a new gasket where it installs in the tank.
If its non of these then I'm afraid that Larry is right.
If you think its a blown head gasket or a cracked block a local shop can do a die test on the overflow tank fluid and see if hydrocarbons ( exhaust gasses) are in the antifreeze. If so then you do have engine problems.
Check the tank fixtures first. I had this happen to my tank. It was a loose hose.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car