I had a similar problem with my 1991 300SL and it looked like a fuel problem so I spend a lot of time learning about and troubleshooting the injection system with no luck.

I noticed that the problem would go away for awhile when I replaced the spark plugs. When I asked why the problem would go away with new plugs then return, one of the guys in the forum advised that the coil is bad and not producing enough voltage. When the plugs are new all of the electrode edges are sharp and square which produces a good enough spark at the lower coil voltage, as the plug electrode wears the edges round off and the spark is reduced.

Thus the lower coil voltage and worn plugs causes the problem. I replaced the coil many miles ago and the problem has never returned.

sldan