Assuming the battery is healthy and fully charged I'd start with some basic checks first. With the c5 off the ground, does the motor and gearbox rotate freely...I'm thinking maybe a fault with the motor ? To protect the control box I'd be tempted to disconnect the motor leads from the control box and directly connect them to a battery .This will show whether the motor and gearbox are behaving themselves. Sometimes the magnets drop and lock the motor, or the brushes fail or stick.
If the motor and gearbox run fine, then you know the problem lies elsewhere.