A speed sensor failure would cause the cart to run at 50% power...
The cart engaging the "roll-away" brake will verify the controller is getting power on Pin-9 and that it has Field driver control and at least the ability to short the armature driver.
Those earlier IQ carts also had a "main-dropout" issue, but that recovers after a pedal cycle.
Here is a diagram for Your cart:
http://www.buggiesgonewild.com/attac...ds_iq_mcor.jpg
The MCOR micro-switch failure is the next best possible cause for Your condition.