It could be a controller problem, or it could be a battery problem, or it could be a throttle (ITS) problem.
Need to do some troubleshooting to know for sure.
However, if upgrading the controller is in your plans for the cart, this might be a good time to do it.
Of course, you have to figure out what the end goal is and then work towards it.
Take a gander at how to determine what your controller/solenoid needs are:
http://www.cartsunlimited.net/Custom_Options.html