I replaced the gears and it still surges.
Brendan, I have read on OGR that some of the earlier Flyonel GPs do have a problem with the gears binding which may cause the surging you are experiencing. These are the GPs with the flat sided can motor mounted horizontally on the power truck. Check the gears on this engine and make sure that you do have a small amount of backlash between the motor gear and the gear on the worm shaft. You might have to shim the motor a slight amount to raise it and free up the mesh.
Ray