My machine started misbehaving while going through the setup procedures, swapping bits and finding the surface etc. When the head moves rather rapidly over to the bit plate and back, the head will jerk in the y-direction. Sometimes it appears to hesitate; sometimes it actually reverses direction for a bit and then continues normally. I ran a carve after it started doing this, and it carved fine. I did not see any jerking while the carve was in progress. But a 3-pass cutout in pine, which I have done successfully many times, resulted in a broken bit. Of course if it jerked while cutting the bit would snap.
I have checked and cleaned the y-axis encoder and replaced the bearings. The gears look good. The belt is clean and tension is set properly. The rails are clean. I checked tightness of the bearings. They seem to run smoothly when the head is pushed by hand.
I suspect a power problem, cables or connectors, or the y-axis motor itself. I'd appreciate any help or insight before I start replacing parts.