I had a similar issue with a Diplomat / MB149 in the past. Many Montblancs have an issue with ink "sweating" in the cap, transfer this to the end of the pen when the cap is posted, then onto your hand and shirt pocket.

How is the inkflow in you pen - do you find it too wet, or does it blot? I agree that if it leaks like that, the problem may be with the nib or feed. Taking it to a dealer may be a good idea. You may also want to send it to a nibmeister like Richard Binder to check out and adjust if needed.