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!!!
---My Previous MBs---
87 300SE (Sold) - Black, Cream Leather
89 300SE (Sold and missed) - Black, Cream Leather
87 420SEL (Died!) - Bronze, Black Leather
---Father's Previous MB's---
2004 S320 CDi - Black, Cream Leather, 18" AMGs
2001 CL500 (Stolen!) - Black, Cream Leather
1999 E55 AMG - Black, Black Leather
1994 500SE - Black, Cream Leather
1990 300SE - Midnight Blue, Cream Leather
1986 420SEL - Diamond Blue, Cream Leather