I had the same problem last fall. It turned out to be the mechanical link where the sensor cable attaches to the fuel pump linkage. The same place where the throttle cable attaches. Passenger side of engine, there was a pin sticking out of the linkage assembly. The throttle position sensor cable is slipped over this pin and secured with a washer and a cotter pin. On our unit the cotter pin hole had worn through and the cotter pin fell out, allowing the sensor cable to slip off (the sensor cable has a oblong hole/slot where the pin fits through allowing it some freedom when the throttle moves to fast for it to keep up). I ended up removing the pin and using a 1/4" bolt with a regular nut to tighten it on the throttle linkage and a second nylock nut to hold the sensor cable on the pin while still letting it slide on the bolt.
It may sound complicated but if you can get a good view of the fuel injection pump linkage you'll see what I mean. I had to remove the back wall of a rear basement compartment to get to it. The linkage has three cables attached to it, the throttle cable, the cruise control cable and the throttle position sensor cable. If not that pin it could be the sensor cable itself.
