If the engine speed isn't changing and the engine isn't stalling, it isn't fuel OR spark. With the throttle plate tipped in, if they aren't varying, the engine will stall. I'd be looking at linkages for something loose, broken, or worn to the point you get intermittent operation.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]