iMac G5 20" 1.8GHz no display, but some time i can see the display but after while every thing get freeze.

This logic boards sometimes gets only two diagnostic lights lit (one and two only.)...but there's no video. The screen remains black. I disassembled & then reassembled both & was able to get one to boot fully (fans running a normal speed, hard drive spinning up) but no video and boot up sound.