Leaf Blowers

I have a YTH20K46-960450004 husq riding mower Kohler 20 riding mower. I started it the other day and it would not move forward or back. Later it moved but now when going up a hill it will free fall rolling down the hill with no brakes. It has nearly killed me because i am going down backwards with no brakes. I replaced the drive belt with a generic but it still does it. If i rock the mower on flat ground back and forth it will catch and move again like nothing is wrong. Help me! Should i replace the pulleys or give up and take it to a dealer and spend $250?