This is a good candidate for the cause of the chain skipping.
The unistrut frame greatly helps getting everything straight and true. In the mean time you might be able to tweak the frame you’ve got to make do. Make sure that the top beam is straight and not bowed or twisted, and that the work surface is flat and not bowed.
Then I would suggest starting with the sled with the weights and router in stalled and lift it by the chains to verify that the sled hangs with its surface about parallel to the frame work surface. If not, adjust the ring until it does. That sets the height above the work surface for the chains. Now look at the arms holding the motor mounts the motor mounts and adjust/alter those to bring the chains parallel to the work surface. Then make sure the motor brackets are very firmly attached so the sprockets are parallel to the work surface.
Those same steps will align the unistrut frame when you build it. Start from balancing the sled and work back to the motors to set the chain height and get the motors aligned. Take a look at the brackets @dlang suggests here as a way to make the unistrut motor mount arms adjustable. Keep us posted on how it’s going!