are you buying the cheap aftermarket starter off ebay? If you are, thats your problem, thse things are trash. I have a bench full of them. They fail inabout a year around here and I love that all my competion uses them.
Simple test: With a volt meter on the battery, start up your cart, make sure the battery is fully charged, if with in a few seconds (10-15) the battery voltage is not at 14-14.5 you have an issue. If at slow speed it is charging at about 12.80- 13.50- and will cimb to proper charge voltage when it is floored for 30 or more seconds, then chances are great it is the starter.
I will repeat my self here.....Dont ever throw away the factory yamaha starter/generator- they can be rebuilt and last another 10 years.
Owner-CARTFIXER in the Villages Florida. Want your golf car fixed correctly- have a golf car mechanic work on it.