Before you start purging and stuff, check the simplest thing, which is the throttle linkage. Have someone else floor the pedal with the car, watch the linkage, does the linkage come within .5mm of the throttle stop on the IP?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.