Hi Cwww,
The first thing I would try to do, would be to disconnect the motor from the screw, and try turning the screw by hand (or with a small wrench, thus getting a "feel" for how the axis is. You may detect a bad bearing, or a gib screw that has come loose, and allowed the gib to wedge into the ways. This has happened to me
It is best to eliminate the possibility of a mechanical overload, before you start replacing motors, etc.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)