[slonomo]

With the cart off, lift the body up and inspect the clutches. Look at the big clutch (primary clutch) and see if the insides of the beveled part (sheaves) are clean. If it's dirty, muddy, or greasy it will cause the belt to slip and not "grab". Also look at the belt. Is it frayed or cracked or worn? The belt should measure at least 1" or 1 1/16" minimum. If it's less then it is worn. The rear clutch (secondary) also needs inspected for dirt, mud, grease. You can run the cart while on jack stands and let the engine run and watch the clutches to see if they engage properly and shift properly. Do a search on youtube and look for "golf cart belt" or "golf cart clutch" and you'll find some videos that'll show you what it looks like when the clutches run properly.