I would put the hood all the way up, so that you can stand in the left doorjamb with one foot on the pedal, engine off, and watch the linkage as you push and release the pedal for any unwanted movement, such as a firewall mounted pivot ball thats worn or the like. if that all looks ok, look at the firewall end of the injector pump, at the bottom where the rear support bracket is, there should be a return spring attached to it that goes up to the arm on the injector pump. Also make sure the linkage isnt dragging on anything like nearby injector lines. I'm sure theres other people with more ideas on this, but this is a good start. It almost sounds like a runaway engine, or something binding up simulating that condition.
just my $.02 worth.
-Chris