Check resistance through forward and reverse switches, I've seen trucks not work with as little as.5ohm. Another possibility is a 1rec. Unplug battery and discharge capacitor, remove 3rec pigtail and motor cable from 1 rec. Keep those ends isolated. Plug in battery and turn on. Check battery volts on both sides of the 1rec. Right side will have battery volts, left side will have EXACTLY half battery volts. If it is not exactly half battery, that is the problem. A bad capacitor can also cause this problem. If you have another similar truck, swapping capacitors is very simple.
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