Strange patterns over several layers of print

[PaulNaude01 0]

Hi 

Does anyone know which setting(s) or conditions can cause or prevent these weird patterns? I find it often but not always and as shown here, on the same part, angle and plane, some areas can have it while others don't.

BTW, the yellow is PLA and the red is PET. Printed months apart, before and after nozzle maintenance and replacements also. I think it only occurs on flat surfaces - can't recall ever seeing it on a curved surface now that I think about it.