I don't know if you managed to solve the problem in the meantime, but I'm experiencing something similar and in my case I noticed that is related to the RAMPS, when it gets hotter, the X and Y axis start to skip steps. Only today I managed to realise that when I did few tests with 1 layer gcode, by replacing the hotend with a pen, and a piece of paper on the hotbed. And the results indicated that after a period when the motors were off, the steppers were working fine, the more I was continuing to use them and the more evident were the skipped steps. I'll try to install some fan on the RAMPS on see if anything change.
Just thought to share it, while I was looking for someone who experiences my same problem.
Just thought to share it, while I was looking for someone who experiences my same problem.