As you know this is a PC based control running on DOS. When I see such behavior, I suspect the CMOS settings for the hard drive, the hard drive and or software, and lastly communication within the control.
Thus I typically check that the CMOS matches the FDISK of the hard drive, not to exceed 504MB. Secondly, I will nuke the hard drive and format it, load DOS and the machine software. This will resolve 95% of the cases. Lastly I will clean the edge connector of the BMDC. PM is very important also. All fans working (including logic power supply) and filters clean.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)