Power Mac - Monitors on G4 MDD 1 Ghz go black after start up

I have a very strange problem.
Here goes:
G4 MDD 1-GHz 2gb ram 2 x120 HD internal
256mb geForce video card
320gb external and a 500gb external
2 LCD Monitors 1x19 1 x 22 both samsung

About a month ago I had the below problem happen once and then it did
not happen again until Saturday morning and then only once and then it
worked fine and I was able to compress the video I was working on. And
when I came in this morning and powered up the Mac it powered up fine
and everything came on fine. The computer is running fine for about 10
minutes and then the monitors go black, This continued all day with it
staying on anywhere from 1 minute to 20 minutes and then you can hear
a fan kick into high gear and then the monitors shut down and go to
black with no video coming to them. The fan quiets down to almost
nothing. The light on the front of the computer stays on and then
starts to slowly blink. All I can do is hold the button down until the
Mac shuts down completely. As soon as it happened the last time I
opened the side to see if the heat sink on the processor or if the
power supply was very hot and they were not very hot at all.

Ok here is what I have tried so far:

Replaced the video card with one out my old G3 that I used in this G4
until I got the 256mb card. (No help sop put 256 card back in)

Tried taking out the ram one stick at a time (No help)

Reset the pram by holding down the command-option-p-r buttons (No Help)

Made sure everything is seated properly

Checked the fans on both the power supply and the big fan for the
processor, I even replaced the fan that came with it with a quiet fan
tonight.

I even tried booting safe mode, with the old fan, and it shut down
right way then

I unhooked all of the peripherals and it ran fine for about 20 - 30
minutes. Then I hooked up my DVCAM deck and it work fine for about 5
minutes as I was opening programs, Photoshop, iTunes the fan went loud
like it was revving up and then the monitor went black.

I then unhooked the DVCAM deck and tried to restart the Mac and it got
all the way up and last about 2 minutes and shut down the monitors
again.

I was thinking of going and buying a replacement battery from radio
shack for the mother board.

I really do not know what else to do I have about 5 project in there
that need to be done ASAP. I was hoping to get them done before
Friday. I am going to a Karate tournament in Daytona and was hoping to
sell these DVDs there.
Any help I can get would greatly appreciated and is badly needed.

Thank you very much
thekaratevideoguy