I have an old rear engine Snapper rider. Model 25086S. While mowing along doing just fine, it will come to a sudden stop. It's like the transmission locks. If I go back and forth from a forward gear to reverse, it will eventually become unlocked and start moving along fine again. I may be able to go 15 minutes before it happens again or maybe only 2 minutes. While working fine, I have all the gears. Any ideas?