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Well you saved me some time. The thin rail I was going to use was a 3/32" oak lath which would have flexed like crazy compared to your 6mm/1/4" acrylic which apparently already flexed too much.
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I will do a couple of your suggestions like clean the belts with acetone and lube axles and such. I've already enlarged the counter sinks because the screw heads were dragging on the left plate...

Your use of the word "creeping" is more accurate than my "slipping". I agree that it may be the CW equivalent of tire squirm. I like your idea of a thin rail. I have several long oak shims left...

First I must give an apology because this is such a long entry. But if you can wade through it you will see that I am at wits end in resolving the problem because I’ve already tried everything...