I can't say for sure, I but would guess that it's due to maneuvers. There are other satellites in Libration Point Orbits that maneuver on the 2-3 week time frame. (They do so no matter what, it's not on an as-needed basis) Also there are a couple of factors associated with maneuvers that can preclude the ability to 'do science' on the spacecraft:
- The spacecraft may have to slew to a different attitude for the maneuver that would point the camera away from the Earth.
- The maneuver itself may cause too much jitter for the camera to take a clear picture.
- There could be exhaust plumes associated with the thruster firings that might blur the imagery.
Most likely it's just the first one, but these are all things I've heard bandied about with regard to other satellites.
For the irregular outages, those could be other satellite maintenance issues, such as momentum unloads, or as you speculated scheduling conflicts, or even hardware issues.
It's not lunar interference, they take images throughout any transits: