sounds like you have a problem in the pot circuit
either a bad pot setting or some shorted wiring somewhere.

you can check the pot values at terminals R4 and R5 (with battery disconnected) and get the potentiometer readings.
with accelerator at creep range it should read around 4800 to 6000 ohms. With accelerator in top speed position it should read around 200 ohms or less.

also check the pot wires for shorts to the frame (ground).

there is also a rapid tune up proceedure that shows the general settings the pots on the control card should be set at from the factory, (these may differ from the lift mfg some)
creep - 7 *
c/a - 7
c/l - 5 1/2
1a time - 4
1a DO - 9
Plug - 8
FW PU - 3 1/2
FW DO - 6

I would pay attention to the creep setting mostly as the truck should only be barely moving or wheels slowly turning (if jacked off the floor). You can try turning this back to 0 and see if there are any changes in the speed.
If no change then as i mentioned above there may be other issues with the pot or its wiring or if everything is ruled out then the card may be bad.