Try a new radiator cap!.
I had the same thing happen to my 560SEC and an Indy MB mechanic happened to witness it and told me to check my radiator cap. I replaced the cap and it has never happened again.

__________________
1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.