I know it sounds crazy but wash the belts in a little soapy water and then rinse in warm water until they feel clean. They sometime put a coating (powder or lube) to keep them from sticking together in storage. You can also use a q-tip or toothbrush and some rubbing alcohol to clean the track on the gear to make sure there are no contaminants there either.

Make sure the wheels spin freely with no belt on them.

there's an adjustment plate on the mech, under the playfield. loosen the two screws and tighten the tension.

bigehrl's right. I just had to do this a couple weeks ago.

look under the PF at the ferris wheel set up. On the left end there are 2 screws. 1 at the top and 1 at the bottom. You can see how the bracket is cut out so you can slide it up and down to adjust the mechanism. Loosen the screws up a little, then pull down on the little lip of the sliding adjuster to tighten it up.

WARNING... don't pull it all the way down. I did that first and it was too much tension and the motor wouldn't spin and the relay kept clicking. I don't know for sure, but I bet that could either burn out the motor or the relay unit. So go a little at a time. Anyway, just pull down some and then tighten it back up. I got mine perfect on my 3rd adjustment. Now the wheels move nice and cleanly with no slipping or jerking.

BTW - Mine is about 1/4" up from the bottom of the bracket.

Let us know this resolved the problem and GL.

another tip, if the above does not work. get some "belt conditioner" at an auto parts store. Spray into a disposable cup and then apply onto the belt with a qtip. Let dry.

I did this on a hurricane with an old belt and never had that problem again,

I could never get mine to work consistently with the urethane belt. It's too hard and too smooth. I vaguely recall someone recommending use of standard white rubber rings instead ... They have enough friction to keep the wheels moving.

Use a big white O-Ring instead of the clear ring that is called for, it is thicker and grips better.