Our 93 300E 3.2L did this, the solution was new spark plugs and the connectors (3) under each coil. Had read many posts and it was often suggested to just change them (Thank you Mr. Dalton!) so I did and the problem was gone!

One of the connectors had a very faint ash gray color where the black rubber ended at the ceramic on the plug. Also the plug had a tan/brown spot on the insulator that the other five did not. Hope this helps.