I've never looked too closely at how the belts are set up on those things... but I have a rough idea as a neighbour has one. I'd wager that something is out of line on this one. Either a pulley is too high or too low relative to the other ones. That's where I'd look anyhow. I know my neighbour is at the point of just scrapping the one he's got... The last number of years he's gone back to cutting with a sickle bar because it's a lot less aggravation.