I had the issue of Polaris 3900 going in circles and staying on the deep end for a while before I figured out the problem.
In some cases the lone wheel connected directly to the gearbox turns faster than the wheels connected to the chain causing it to veer right because of slippage.
When this happens check to see if the chain is secure. It should be tight enough so that the tensioner does not touch the wheel sprocket. In my case I had to remove some 6 links from the chain and once I did this it started working well.
When you disassemble the unit you will see a sticker with a diagram of the drive chain on the feedpipe. Check that the angle of the tensioner is consistent with the diagram so that it does not touch the sprocket.