imac g5 has trouble booting

[laser nerd]

imac g5 20" 1.8 Ghz . When I start the machine it begins the process then sudenly stops . If I keep on trying to boot up it will eventualy boot up after about 10 atemts but will have horizontal lines like interference . The interference will disapear over about 2 minutes and the computer works fine . It will boot from disc or in target mode no problem . If I start it in target mode and run it for about 10 minutes then restart it works fine . Any help would be great . Thanks Rick

 

[Satcomer]

When you boot from target disk mode run Disk Utility on the iMac's drive to see if it went south.

 

[djackmac]

More than likely blown/bulging caps or else a bad GPU showing the symptoms when cold. Its only going to get worse.