Sounds like you mixture is rich and you need to adjust your on/off ratio. Lots of posts on how to do this with a dwell meter or DMM with a duty cycle. Maybe do this after you've replaces the O2 sensor as JNT suggests.

Other causes, less likely, are a sticking plunger in the fuel distributor, or a bad temp sensor, telling the computer to enrich because it thinks the engine is still warming up.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe