One thing I found was that I hadn’t tightened the Z Motor/Idler tight enough and the belt would skip on a +Z move. I found that using carpenter’s shims (wooden shims used to level various home constructions) fit very nicely between the Z and X Motors and, loosening the Z Motor mounts slightly and jamming a pair of shims between the two motors allowed me to tighten the belt so it was ‘twang’ tight and no more skipping!
Tom
PS: This should be a tip in the construction guide…