I'd be looking to fix all the slop in the throttle linkage and find out why the IP lever won't hit the full travel stop. If you're not getting full travel when you push the lever manually and you also have a bunch of slop in the pedal linkage, you're probably getting even less throttle from the pedal. Lack of full travel = lack of full power. On an engine that had ~65HP when new, you need all the power you can get.

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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]