Noisy Z-Axis

For some time,  I have had intermittent problems with the Z axis.  It would make horrible noises and not move,  or very reluctantly move.  I assumed that running it without sufficient lubricant on the lead screw was the cause.   I feared that something nasty was going on in there.   I rewrote several of my programs to give me more control over there Z axis feed rate.   I finally reached Rhee's punt that I could nor ignore it anymore.

The good news is that I when I finally removed the stepper motor, I could see that the set screw that locks the motor shaft to the coupler that drives the lead screw was not in its hole.  That or worked at low speed was probably mostly friction between motor shaft and coupler.   After hunting around in an awkward position inside a black motor mount I finally found the black 10-32 x 1/4" set screw.  The hole was slightly cross-threaded, so I rapped it, screwed it in and everything works like new!