Takes several attempts to boot.

I'm a newbie tech, that is trying to troubleshoot a problem with an older custom built system. It has a Supermicro P6SBA MB, Pentium III, 384 RAM, PS2 KB/M, Printer, USB Scanner, USB Micrscope attached. The problem is that the computer can take several attempts before it will boot-up. The monitor shows that it is off the hyway, until it boots, then all is fine, and works fine until it's shut down and restarted. Every so often, it will boot on the first attempt. It shows power, green light, but the HDD light (red) will stay lit solid, not flickering. There has been instances where it will lock up prior to loading up the complete desktop, like just prior to the windows opening sound. I'm thinking along the lines of a weak Power Supply, or maybe a faulty Processor. Any help would be grateful. Thanks