Short horizontal lines appear across the screen in quick flashes when using radeon and 2.6.32-5-686. I tried liquorix 2.6.36-1.dmz.1-liquorix-686 and get no interference, so assume the problem is the radeon driver's interaction with the debian 2.6.32-5-686 kernel. If I install fglrx, I get no screen interference with kernel 2.6.32-5-686.
Hope that makes sense....