Has this been going on ever since the pace unit was installed or is it something that has just happened lately?
I am betting the wire harness was added in a manner that pulls on the wires when the pedal is pushed to WOT by foot, which is just a bit more than when you push the pedal by hand. Confirm the wire harness does not route under or pull on the accel pedal linkage at all. if that doesn't cure it, then swap pieces of the harness from the accel pedal to the pace unit. in my experience this has been caused by not allowing enough wire length in the harness before cutting into the OEM harness to tie into/add the Pace unit.