How long and how thick are the horizontal lines? What color?
Occasional very thin usually short discolored horizontal lines at a top, bottom, or diagonal edge of color patches suggest "the chroma bug" in a DVD player although I am told the Panasonic RP91 does not have this problem. Some of the poorer progressive scan conversions hide the chroma bug. Proliferous uniformly spaced thin horizontal lines at the left and right edges of moving objects are a sign of progressive scan conversion poorer in a different way.
Turning off the progressive at the player generally causes the projector to automatically turn its progressive conversion on, and vice versa. Whichever you choose depends on which looks better.
What happens if you shut off the progressive on the projector manually?
Alternatively, does your projector have a requirement as opposed to an option that you turn progressive scan on or off to match the source?
Video hints:
http://members.aol.com/ajaynejr/vidbug2.htm