Just read a book about the changes made to the '90's' series SL and the transmission has goveners which retard the timing and fuel if certain things do not respond ie; over rev and over speed. This as well might be worth a look even though you didn't go over those limits. Also there are two fuel pumps located just near the passengers rear wheel (under a black plastic cover) There is a presure regulator with those. Also check the distributor cap for cracks and risidual dust (carbon) this will retard the firing.
Let us know what you do find. Thanks