Power Mac G5 won't start up

Power Mac G5 Dual 1.8 GHz June, 2004 2GB RAM 1GB HD

Will not start normally. Typically, crashes on startup with the "You need to restart your computer" power button graphic and kiss of death message.

If I connect an external startup drive, hold down option, and wait for the options to appear, I can choose an external startup drive. It started twice this way, but froze just a minute or so later. Most of the time, it just freezes after I select the drive. The Apple logo appears, then a black box appears where the "Please restart your computer" message should be.

I've tried: Unplugging the AC cord for 20 minutes.... Resetting the Parameter Ram... Disconnecting all peripherals... Re-seating all the memory sticks... Vacuuming up as much visible dust as I can...

I have not changed the backup battery, but it held the clock setting with the computer unplugged. I checked the time/date once when I got it to start briefly on the external drive.

It does not start up from a CD or the internal drive... Either it displays the circle with a line through it, or it gives the "You need to restart your computer" message. Then the fans take off like a jet's engines preparing for take-off.

Any ideas?