I have had similar problems in 3 of my printers over the past months after long periods of heavy use. This was the extruder under-extruding (missing steps as I found out) on one printer, and the X or Y stepper missing steps (print suddenly shift several cm in x or y direction) on the the other two.

In all three cases it was the stepper motot cable that needed replacing. Apparently these cables are not up to continuous bending at stress points (cable ties used to fixate them) as the extruder head moves around.....

Replacing the cables with brand new ones resolved the issues in all 3 printers. I now do not use tight cable tie fixing for the cables but allow them to move more freely, preventing development (I hope) of weak spots in the cables.