Every 20 or every 40? And is the band on those layers? If so then something with the solid layer is causing it.

Knowing black PETG, and knowing that lower temps reduce the gloss, I'd say that the solid layers are extruding much more and the nozzle is not keeping the temp up, thus causing less gloss. So you could try to increase temps overall, or check to make sure the nozzle stays at temp on those layers, or slow down speed which allows the nozzle to keep the temp up, or of course stop using solid layers.