There are several problems that can cause your problem as follows; the controller, throttle, internal battery conections, battery contactors, wiring harnesses and even a motor problem. First off how old is the bike? If it's still under warranty talk to Currie. Actually Currie maybe a good place to start anyway. But considering you have 2 bikes you should be able to find the problem very easily by swapping parts between bikes. I would start by looking at all the connections be be sure they are not coming loose. Then swap one part (maybe the throttle of controller). If the problem stays with the original bike swap another and so on till you find the problem.