iMac g5 booting and screen problems

[Philbee]

Right here we go, I purchased an iMac g5 2gb ram 1.8 a couple of days ago, and when I recieved it plugged it in and everything was running fine, this morning I switched it on and it started to boot up to the apple logo then these weird stripes started to cover the screen and it just wouldn't boot up and went to the blue screen,so I left it a while and tried again it booted got to the normal screen when loaded then started playing up again ! Really dodgy looking stripes and pixilation, so... Any one got any ideas? Is it a video card problem? Or something much worse? Btw it won't boot at all now just apple logo pixilating then blue screen :-( HELP