Speed Gradually Increases Or Decreases After Speed Is Set

Verify engine performs normally. Ensure accelerator pedal and linkage open throttle fully.
Test vacuum dump valve. See VACUUM DUMP VALVE TEST . If vacuum dump valve is okay, go to next step. Replace vacuum dump valve if necessary.
Test servo. See SERVO TEST . Replace servo if defective. If servo is okay, temporarily substitute known good amplifier, then retest system. If amplifier is okay, recheck all electrical and vacuum connections.