This fixture can flash upon new installation due to reflected light bouncing into the photocell. Depending on where it's mounted, this can be corrected by either:
1) Re-aiming the light heads so that the light is aimed differently and won't bounce back into the photocell.
2) Re-aiming the motion sensor box (on the MO model) so that the photocell isn't catching the light.
3) finding what surface the light is bouncing from and either moving or cover that (such as a car parked directly below the fixture).