Diagnosing Belt Skip Under Load

[philso]

It looks like the hex shaft that your arm pivots on is supported by one of the green plates on each side. Is it possible that those plates are flexing? It is not clear from the angle that the video was shot at if there is room to add the brace that Kevin suggested. There appear to be some spacers between the sprocket on the hex shaft and the green plates on each side but I could get a good view of the gearbox end.

We are fighting a similar issue but in our case, the torque on the arm is probably causing the center-to-center distance of our gears to increase, leading to the gears skipping. We have our VP mounted on a rather thin plate that is only supported on one side allowing it to flex under load. We may end up using the same solution that Kevin suggested for you.