You missed the Popsicle sticks.
This message was sent from my Android phone so if it has the wrong version of a word(s), the wrong word(s) altogether or is just plain gibberish then it probably isn’t what I typed. I actually know the difference between “there”, “their” and “they’re” but the phone doesn’t 
I would second @madgrizzle 's comment. It looks like it is a combination of problem areas. First, one of the chains is near vertical. There is very little tension in the other chain. When it begins moving down, the second chain is completely loose, and the roller drops on the ring. This pulls the sled to the left, and creates a jag in the cut. This is compounded by the friction between the ring and the pulleys.
I would like to contribute one more thought on @Axel 's issue. It looks like the rollers could be pretty heavy, since they are made out of steel. It imparts a downward force. When the chain is very loose, like it is in the video, there isn’t enough tension to hold it up. At some point, the tension is low enough that the downward force causes the roller to drop quite a ways on the ring.
Have you tried more weight? I have heard arguments that it should not matter, but if the bit is spinning and pushing back against the gravity pull on the sled, it could wonder. I have experienced this when I tried to ‘lose the bricks’ when in initial beta. It could not hurt to try and add some weight to the sled and try that same cut. If it works, yeah, if not, we know!. Love the sled anyway! Nice job.
more weight would make a big difference down in the bottom corners, but up in
the upper area, it really should not matter (although, as Lon says, tests trump
theory).
a video of it as it’s misbehaving would help a lot.