Pal games in Progressive scan, problem

Most of my PAL games work fine on my modded Xbox in 480 progressive scan, and they look fab.

The odd game, Fable in particular has faint virtical lines about 1cm apart covering the whole screen.

Is this because its a PAL game, or is my Xbox playing up. If I get the NTSC version will it be OK ?.

If I switch my Xbox back to normal Pal, the lines dissapear, but obviously no 480p.

HELP