B737 Recirculation Fans.

[B737NG_Pilot]

During various phases, of flight, these Fans turn OFF and come back ON, automatically? Why?

[STBYRUD]

Huh? Normally they don't! They both (unless there only is one like on the -700) always operate unless someone (hopefully not to have a smoke in the cockpit :=) set a pack to high flow or turned them off manually. Do you have a concrete example?

[BARKINGMAD]

Are you getting a flight deck warning/alert that this is happening?

Often the fwd galley ovens sound like a major rotating assembly running down when their fans switch off!

If you are detecting such an audio cue, next time phone cc and check with them regarding oven use. :)