Any coolant purging from the overflow tank on right side just behind the front wheel? If so, then the cap is probably the culprit. If not, it sounds to me like a bad monovalve with the way your heat works intermittently. If this is the case, depending on model year of your 300E you can repair or you must replace the monovalve to solve problem.

Good luck,

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi