I keep a spare carriage assembly so that I can just swap a new one in when the old one gets worn. I can then rebuild at my leisure. I popped in a new carriage and all of a sudden my print which consists of 5 pieces has the layers shifting to the right. It was a bad x stepper. Once replaced my prints returned to normal. I can not figure out how a stepper could do this, never the less it did. I replaced with an identical stepper. I decided to figure out what went bad in the stepper as I have more of them. First I printed my parts at a reduced flow of 20% to see if the stepper had a drive problem. Sure enough I print the part with no shift letting me know that it was a resistance problem. I find that the printed filament itself offers resistance to the print head. Next thing I did was take the bad stepper apart and a good one apart. I swapped the bearings in the tops of the two steppers.and put the bad stepper back in. Now it worked fine. I put the previously good stepper with the bearing from the bad stepper back in the machine and my problem returned. I now knew it was a bad bearing in the stepper. I still do not know how it could shift the print in only one direction. Steppers are cheap and it is not worth finding a replacement bearing.