Z axis/filament problem

Aright, so I have looked around and haven’t seen anyone with this problem yet. I keep getting the gaps in the print as the print gets toward the top. I’m printing in PLA and the heated bed is around 60 C. The Z axis is calibrated, and it is fine toward the bottom, but when it gets about 10 cm up, every time there is a noticeable horizontal line going across. Does anyone know what could be going on? Below are some pictures of what I am talking about.

Is that vertical gap supposed to be part of the model?

It looks like you may be under-extruding… Make sure the z-axis threaded rod is lubed well… Especially at the point where the horizontal lines start appearing.

Likely under-extrusion, as the previous poster noted. Try turning up the flow rate slightly.

However, it seems to me there are random gaps throughout the part. Bad filament could be the cause. If some filament regions are larger/smaller (measure with calipers), or stiffer/flexible, under-extrusion may occur at random. Again, the solution would be to increase flow rate slightly, until the gaps go away.

What filament brand are you using?