iMac G5 from 2006 gen will not power up!

Hi,

2006 Gen iMac G5 will not power up! It will show a black screen for a min then will produce a loud fan noise. I have checked all capacitors and they are fine. The RAM works ok too. On occasion I get a message that pops up and says "You need to restart your computer. Hold down the power button for several seconds or press the restart button". I will restart but will be back to square one. The problem looks like this:http://www.youtube.com/watch?v=tBe8v4gycOE . The message is this one: http://farm6.static.flickr.com/5122/5232814920_db1c169b0d.jpg

Many thanks for help!