I had a very similar problem turned out the old ATX computer power supply was inadequate for my power needs. I discover the solution to the problem when switch to PLA, and could get away without using the heated bed. my layers shifted slightly back and forth through out the print though unlike the pictures above, where they appear to be shifting in one direction near the end of the print. Got a 12V dedicated power supply on the way to solve the problem.
Posts made by Steevo4321
RE: How to Fix Shifting Layers