Flickering when playing NTSC games on PAL DC

[gummi]

On occasion, when I use my boot-cd, the top 10% of the screen is flickering. It dosn't happen all the time but it does happen fairly regularly. Turning the machine on and off a few times seems to fix it, but I would like to know whats causing it. I'm using a PAL DC, and the problem only occurs with games which don't have a 50mhz PAL option - so they are running at NTSC speeds.
If anyone has had this problem before, or has any idea what is causing it then I would greatly appreciate any help.

thanks,
gummi