Thanks for your reply, Zach.
I’m back in my shop this morning and am checking a few things. From what you said, my belt tension should be OK.
When I experienced the problem last Friday, I took my gantry off the assembly, removed the pulleys and checked for slippage. I didn’t see any signs of slippage, but I ground some flats onto the motor shafts to help ensure no slippage. (Yeah, I know - I should have read more deeply in the instructions to start with). After doing all of that, I started my job again and still got slippage that sounded like belts jumping. I had other things to do, so I decided the best thing was to let this issue slide and go mow my lawn!
Over the weekend, I read more about motor current and how that can lead to a loss of steps. I’ve been checking that aspect this morning. Based on what I read, I raised the voltage read on the X and Y trimmer pots to 2V from the approximately 1.3V they were set originally. This is based on information I read in the Shapeoko Wiki and forum. I have NEMA 23 motors on my X and Y axes; NEMA 17 on Z. Does the adjustment to increase motor current sound right to you?