if it hums but the hum does not speed up when you start pressing the pedal right after you make the start switch in the accelerator control (creep mode) then it sounds to me like the pot circuit (a bad pot maybe).
What does the 1C cap look like? is it bulged out any around the terminals on top? you may want to check the cap and make sure its charging and discharging correctly. There is a test proceedure in section 4 of that troubleshooting guide that tells you how to do this.

one other thing that just came to mind that i have ran into before is the plug on the back of the card, make sure all the wires are connected good and you can also put a slight twist on the blades to make a more positive connection. You can remove the plastic cover off the plug and see where the wires connect through the plug.