Screen Blanks shortly after power up

When powering on the PC, the screen is fine for about a minute. Then the screen goes blank and the power indicator on the monitor turns from green to amber. All indicators on the PC indicate everything is OK. No spare monitor to switch to. Running an AMD XP 1800, 1gb RAM, an 80 and 120 GB hard drive, CDROM, DVDRW, LAN PCI card, and Windows XP Pro. I am leaning towards a bad or incorrect power supply.