I had exactly the same problem. All of a sudden I got shifted layers on my delta printer, all in the same direction (for me it was towards the z-tower). I tried to tighten belts, lower/higher stepper driver current and different firmware, both Marlin and Repetier, nothing worked. What I did in the end was that I moved the stepper driver for my z-tower to my empty slot (which normally is for a second extruder on RAMPS 1.4) and reconfigured the "pin.h" file in the firmware so that my z-tower motor now is driven from the "E1" slot on the RAMPS board. That solved my problem! I guess it was a hardware problem with either the Arduino or RAMPS board.