When backing slowly Cart will jump into forward quickly

[clown9644]

When I am backing out of my drive, slowly and sometimes feathering the throttle, the cart will violently go into forward. When I remove my foot from throttle it stops and I then can back up again. The lever is in reverse and does not move. Where do I start looking for the issue? For the record: I bought the cart used and it may be made up of several different cart's parts, something I learned about the dealer after the purchase. I have no idea of the age of any of the components. Sorry for my ignorance--first cart.