Needing Help with Power Issue

I have a Visionman Server, model V133-8212, which I have had for about 4 years. At the time, I purchased two of these servers in case one ever went down I would have parts to stay up and running in our dental office. And that is exactly what happened last week. The one server I was using as a server (running Windows Server 2003) starting rebooting itself about every 3-4 hours. Over the course of a week, the time between reboots decreased, and at times would not even get through the entire startup process before restarting again. After ruling out battery backup and other potential power input issues, I moved on the the second machine I bought (once again, identical to the first) which I was currently using as a workstation. I swapped out power supplies (by Orion), and still got the same reboot issue. So I moved on to swapping out memory; still the same. Lastly, I moved the hard drives from the original server to the second one, and things have been great.

So my question is - could the power problem/reboot issue have been caused by a faulty motherboard? If so, it is a Biostar P4M900-M7 SE, is it worth replacing into the faulty machine? I can get one for around $110. Or could the issue be somewhere else?

I would appreciate any comments and/or advice.

Thanks!

John York