ditto... had a similar problem on my tibook 1ghz. unfortunately for me the computer would lock-up so even an external monitor wouldn't help. i left Console and Process Viewer running hoping i could find the source - no luck. my problem was intermittent so i was able to run the hardware test - everything passed.... but still horizontal lines and lockup, even when rebooting to a separate partition. i was sure it was some harware failure. i pulled out my apple warranty stuff and used the System Profiler to check my serial number. while doing that i happened to notice the logs pane. under Console Log there was a reference to:
CFPropertyListCreateFromXMLData():plist parse failed;the data is not proper UTF-8. The file name for this Contents/Info.plist --
file://localhost/System/Library/Extensions/ATIRadeon8500.kext The parser will retry as in 10.1, but the problem should be corrected in the plist
well i couldn't find the proper plist (mac novice) so i wandered into that extension folder to see what it is. i know plist items can be looked at/modified by textedit, so i double click the ATIRadeon8500.kext and i'm promted to pick an application to open it. i know it's not a plist but i choose textedit anyway- wham all my kext icons turn to folders. i reboot and no more flicker no more lockups. hopefully someone more knowledgeable can shed some light on this, but i've been running 7 days straight - good as new (knock on titanium)