I have a very old Taz 6 and no matter what I try, I cannot get a successful print. There are two things thats happen, either:
Each layer of the print is slightly rotated from the last one, so instead of a straight up rectangle it would print a spiral, or
The nozzle would not move up between layers, so eventually the nozzle would be melting the print and moving straight through it while printing another layer. We tried to level the z axis of the bed but no results.
Have you ever gotten a good print? Are you driving the print from the SD card or over USB? What is your process flow to produce the gcode? You may have a bad gcode rendering. If your shift problem occurs at different times during the print, perhaps board or harness noise (ie loose connections).
Have you tried reflashing the firmware? Sometimes “resetting” the system with new firmware (even if overrighting the same firmware helps. This would wipe out old, potentially corrupted firmware.