it could be a few things First mark your material to a chuck jaw or a letter on the collett. Make sure the material is not slipping. second might be spindle belts slipping your not telling me the size of part or material but too much tool load could slip the belts.
Third i believe this machine has a taco-generator for spindle feed back thias would be a smaller belt coming off the spindle pullys going to what looks like a small motor near the spindle. check for belt slippage there. also this tac can be cleaned the brushes get dirty. you should be able to tell by watching the RPM display on the screen if you tell it 500 RPM and the screen says 515 thats ok as long as it does not change from 510-520 if your rpm flutters too much your tac is dirty. however from you saying it is only the first or second pass that misses then i would look at a slipping part or belt.