I had the same symptoms afetr running at speed for about 30 minutes.
My garage told me the same, wear in the distributor was casuing the misfire and electronic ignition would assist in curing this.
I opted for both changing the distributor and EI.
The fault cleared up completely although I did get another misfire which was attributed to a sticking valve, the one that sits on top on the inlet manifold, sorry dont know the technical name.
They cleaned this and all misfires and staring problems have gone away. For good??? who knows.
Rob Walker
1968 280SL papyrus white/green leather
2004 230 SLK silver