My setup is a plasma gantry, A axis slaved to Y. Leadshine microsteppers, PMDX-122 breakout board.  My A axis is missing steps, and sometimes stalling but only in one direction.  Initially the problem developed when a program was finished and the gantry was heading for home.  The A axis would quit (just for a couple of steps)for a better word and the gantry would see/saw its way back to home.  Now the problem is happening during the program as well. No whining from the motor, just makes a clunk like a gear skipping on the rack and then resumes. I disconnected drive belt from drive and ran motor by itself - same issue. Misses steps only in one direction.  Travel speed is 250 in/min, accel 12 so I don't think I'm exceeding the motors capacity. Same motors on my X and Y axis with same settings - no problems. This has developed it seems over time.  I have been cutting parts for about three weeks now. Checked my connections, all seem OK.  Anybody have any thoughts.

Try swapping the motors to different axis to see if it follows, also try putting swapping drives. Just do one at a time and see what happens.
Hood

I would try swapping the motors between the two drivers.  I think you'll find the problem follows the driver, not the motor.  Then try swapping the connections from the PC to the drivers.  My guess is you'll find the problem follows the PC connections.  What are you using as an interface (breakout board?  If so, which one?) between the PC and the drivers?

Regards,
Ray L.