Could it be retraction problems or filament slipping or extruder missing steps?
Depending on slicer settings the perimeter is printed first, then infill, then retraction, z moves, then petimeters again. The retraction could loosd steps and stop extruding until infill is reached, making it appear the outer shell isn't printed. Check if the printer is going through the motions of printing the missing walls/perimeters.
Did you have a look at the g code? Did you observe or took a video when this happens, found anything of note? Try to isolate the problem and have a smaller print in a similar way so it's faster to try things and test it.
You could try reducing retraction velicity and acceleration (check jerk too) to see if that make any difference.
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Kind regards
Imqqmi
NFAN CoreXY printer:
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reprap.org]