[Wlug] reboot problem

At the meeting before last, I asked about a problem I'm having with the computer occasionally going to a black screen with no disk activity on reboot, right after the grub menu goes away. Overnight memtest showed no errors, I put a different vid card in, and still had the black screen problem. One of the things someone mentioned was to test the power supply. Recently, I had a few hard lockups and a couple of spontaneous reboots. So, how do I test my power supply, other than replacing it and seeing if the problem goes away? Or is that evidence enough to change it ASAP? According to lm-sensors, voltages are fine, and I've even sat and watched gkrellm for long periods of time - they look stable. Any suggestions would be appreciated. Greg