I am thinking that this may be a possibility - although I am not sure exactly what you mean by “over extruding” or “Z too low”. I tend to run default settings as my experience is low. Over time, the outside of the extrusion part of the head does get a bit crusty. I have cleaned it once. Perhaps I should increase the cleaning rate in my operations.
Anyway, the problem occurs when a layer is being formed over infill latticework near completion of the top of an object.
I did do some experimenting with cleaning the head internally as mentioned in the first reply (interesting results!) but I suspect that clogging artifacts of this type would appear early in the print cycle.
Thanks so far to everyone who has responded!