A)Over extrusion, nozzle catching on extra material in the Z space
B)Stepper drivers skipping steps and temporarily shutting down – running too hot, insufficient cooling
C)Slop in gantry assembly
D)Z Axis moving less than required. – IE steps per mm not set correctly or microstepping jumpers not set correctly – printing thinks its moving say 1mm down but actual moves half or double that – causing over extrusion see above
E)X or Y stepper motors “skipping steps” (usually because of A or . This should be evident in the layer shift – has it shifted in X or Y?
The way to diagnose – show me an entire print which has skipped steps from side on noting which is X+ and Y+ so I can make an accurate diagnosis – or just look yourself. For each individual shift – does the shift occur in one direction? Which direction. You’re unlikely to get something which appears in both at the same time. Look closely at your print.
I didn’t see any pictures posted here of the most recent tests i requested a few messages ago. You were going to print a test cube without heated bed from memory.
Especially for diagnosing shifts, some clear side and front photos of the entire object from side view are critical.