I second about the motor coupling. I was loosing position on my Z axis. I tightend the coupler and I was still getting slippage.
After removing the coupler I could see that the coupler ID had a very rough finish and keyways are not used.
So smoothed all the couplers IDs with a sandpaper roll to knock down the high spots. I have not had a problem with holding position since.
IMO, I don't think it is missed steps. I have spent alot of testing maximum axis motor speed on my series 1 machine.
What I see happening is the steppers work fine till I hit the speed threshold (no cutting loads). Then the stepper gives it up and just buzzes with jerky movement or moves alittle and jerks around again. It's very repeatable within a few IPMs. Although I did get it to run at 200 IPM for about 5 seconds once.
What I'm getting at is the stepper doesn't miss a step here or there and is hard to notice. It makes quite abit of noise and is pretty easy to tell.
Tormach PCNC1100, Mach 3 R3.043.037, MastercamX5 level 3.