A little more info would help. Does it do this w/ the tines in the ground or just running free above ground? If it does it in the ground the belt is probably too loose. If above ground, it's probably a major problem in the tine gear box.
Mike

It does it when the tines are in the ground and the belt is burning on the pulley trying to make it turn, then the engage lever will pop up, then when I reengage the tines they willl work fine for a while then it does it again. It seems to work fine when the tines are above ground.

Assuming everything is OK in the tine gear box including the oil level, then you need to adjust the belt tension w/ the belt adjustment block.
Mike