I made a little progress on the Shapeoko today. The Y axis motor had been stalling and humming instead of turning. I disconnected then reconnected it. Everything started working again until I got about halfway through a test job, and the same thing happened. Since the motors always seem to stall partway through a job, I think the problem is that the drivers are overheating. I turned the current pots down as low as they would go before the motors stopped turning, then bought some heat sinks on Amazon that I hope will keep the drivers cool.
Hopefully once the heat sinks are in place everything will be good to go. I want to get to work making stuff already!