Thanks again everyone.

Once I have the OK from the manufacturer I'll take the drive off and run some tests to exclude it as an option.

JazzCNC - What you're saying makes total sense. It seemed so odd to me that the drive would stall that easily when the current was lowered and there was no load. Makes total sense that something else is causing friction. Couple quick things:

1) Is there anything I could have done which could have caused this? The slider has hit the end a few times while I was getting used to the software but nothing major.
2) Would this be obvious when I take it apart or could it be a subtle bend?
3) How would you test for it?

I'm just thinking about what the manufacturer will ask and what I'll need to come back with. Sadly, my experience with Chinese companies on sires like AliExpress is that they don't like to take responsibility when there's a problem. Hopefully this will be different.

Finally, if you're correct in your assumption, is it easy to fix?