This problem unfortunately is an internal timing issue. It occurs when one of the plastic timing gears loses its alignment by jumping a tooth. It can also happen due to wear if used extensively. Or it may have been a defective gear that now showed its failure due to weakness in the part.
his one needs the services of an experienced repair technician because this one requires specialized gauges to re align the parts once the gear is replaced.
Most vacuum repair shops also fix these as they usually do both vacuums and sewing machines. I would check locally for one. Its not an expensive repair just needs an experienced technician to complete this repair.
I wish I had better news for you , I really do. But it will be a quick fix for them ;-)
In any case, I will stick with you until this is complete.
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