Bad layers in otherwise good prints
This can be caused by various problems, of which an aforementioned filament diameter is only one of multiple possible causes.

The observed phenomenon is called under-extrusion. Temporary under-extrusion can also be caused by filament friction on the spool, too low or too high filament extruder gear friction (pressure on the filament by the roller), or friction in a Bowden tube (dust particles). 

Please ensure your spool unrolls freely (not entangled; entangled filament is audible like "ker-chunk" when the filament is rearranged on the spool by force of the extruder pulling) and check the extruder for friction or slip. If the extruder gear slips, this is also accompanied by an audible "click" that is called clicking. This generally implies that there is too much friction in the hot end, or an obstruction/clog.

Considering that your print recovers from the problem, and that it occurs randomly, the best guess is that your filament in entangled now and then.