It could be one of two things; either the belt is slipping on your centrifugal clutch, or there is a worn contact on your switch plate. Have a qualified person look at your belt (most golf courses have a mechanic or golf cart repairman) who should be able to help out. If there is glazing on the inside of the belt, that would cause the belt to slip on the centrifugal clutch. That probably happens more than you realize, but is especially noticeable going uphill.
