Hey, Guys - after hundred of successful prints on the R1, I happened to get a small layer shift in "X" only (see pictures) and after many hours of trying to debug, I found that the solution was the locknut on the heatbreak.
The hole for the heatbreak in the aluminum block is larger than the heatbreak itself. The setscrew forces the heatbreak to the back of the hole, which eliminates movement in "Y", but you will find that you can move the hotend "sideways" a bit - or "X".
So, when I was printing a part that produced a bit of "slag" during the print, the nozzle hit this on the next layer and moved the hotend.
The solution is the locknut, slightly snug up against the bearing block.
All the best, Don