I have exactly the same problem with my 1989 300se. It used to happen 2-3 times a day, but it seems to have calmed down, and now it happens maybe once a week.
I noticed it coincided with wet weather for me, i'm not sure if something got wet that shouldn't have, and is now drying out or something.
I've had the o2 sensor adjusted, oil and filters changed, and it hasn't really made much difference.
Sorry I couldn't help, I just thought you'd like to know that someone is sharing your pain!!!

Cheers

__________________
Oliver