'''Connect to an external monitior.''' If the lines don't go away you have a video card problem [http://www.everymac.com/systems/apple/imac/faq_g5/imac-g5-video-upgrade-hack-to-support-dual-displays-spanning.html|dealing with this issue] is an advanced DIY project.
If the lines do go away the problem could one of a couple internal components... a cable or the LCD.
The freezing could be your CPU or RAM.
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