yes i had the same problem last week. see post
Cutting went crazy, normal, crazy again
I found wrong was only ONE of the Y belts had gotten loose. It would run find some of the time when go crazy, the forum help me with a lot of troubleshooting and turn out that the right size belt got loose, I was checking the belt on the left size because is close to the area i was working in.
So read that post for more troubleshooting, but i would start with the belts. Good luck