An excellent example of intermittent contrail formation was posted by GeoengineeringWatch.org:
Link to Original video here.
The narrator blames it on spraying, but I think the clue is in the image at the top of this post - perpendicular to the trail are five parallel clouds that look like the result of rotors or wave in the air that often cause rippled clouds or rows of clouds.
Image Source: https://cloudatlas.wmo.int/orographic-influence-on-the-leeward-side.html
This suggests the contrail (which I think is largely aerodynamic) is flying though the invisible portion of the rotors or lee waves and the on-off is the result of the variations in the air between the up and down portions of the waves.