Although you MAY have PCV problem, but the defect oil cap on the W124 is a well know problem and that is why Mercedes released a improved version for it (see my original answer to your question).
I quote a written line from a MB tech with 30 year experience on this:
"Mercedes has a new style oil cap which does not leak - for all models at about six dollars. This is a must-have item. "
So go buy one and see if it still leaks.