|Could be a failing power supply or bad capacitors. Look at the link below for more on bad caps. |
One other cause could be bad contact on some of the hardware. Remove and reinstall all the cables, cards and memory. Remove and insert the memory 4 or 5 times to burnish the contacts.
Some of the power connectors have locks on the plug that you need to depress in order to remove. Don't break anything and exercise proper static precautions.