I had this same problem with a 1974 450SE. There is a cold start enrichment valve which is probably remaining open after the motor heats up - thereby continuing with an overly rich mixture which fouls the plugs. Check the plugs - if they are gas fouled it may be your problem. The valve was cheap and easy to replace.

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Abe
1970 280SL
1974 450SE
1998 E430
2000 E55