I am suddenly having X-Axis missed steps issues. I tore down the X-Axis to check for problems, I swapped stepper motors, I changed outputs between X & Y axes on BOB, but it still misses steps on the return to zero. I slowed the motor tuning settings to 100 ipm. I lowered acceleration to 5. I turned off tool path view. I changed the parallel cable. I tightened all screws on electronics. I am rastering a 6"x26" dome-shaped object and it started missing steps on the return leg towards zero on every pass - thus making a parallelagram. Then it would go fine for 15 minutes & suddenly loose a bunch of steps and be off by 1-2 inches as if a lot of skipped steps. I have been repeating the same program dozens of times, & it fails in different spots each time now. The last time it was on the return leg to zero on X when it stopped X travel, suddenly rose about 2-3" on Z, moved on towards zero, then lowered back to surface.I stopped the program & it thought it was 2 inches further down the X Axis. I checked gcode - the endmill was not where the gcode said it should be. I was as if the gcode kept moving in the X direction while the mill was moving in the Z direction up & down. The errors are so random, I am thinking about getting a different BOB next. I am running ssd computer with WinXP Mach3 R3.043.066 for some time now without problems. Any suggestions would be greatly appreciated.