Help needed with random system pause issue

My brother's desktop system is experiencing some strange behavior. Randomly, the system will pause or appear to "hang" and then it will come back and be responsive again. He's running Windows 7 Enterprise edition (32-bit) on a Dell Optiplex 755 with an Intel Core 2 Quad CPU, 4GB of RAM and 2TB of hard drive space (2 250GB HDDs and one 1.5TB HDD).

I've run malware/virus scans and nothing's been found.

I've tested the RAM using Memtest86 and no problems were found.

I've run hard drive tests using SeaTools for DOS (booted from a CD) on all three hard drives and no problems were found. I ran the "long" tests on all three drives.

I've run the built-in Dell hardware diagnostics and no problems were found.

I ran a chkdsk on all hard drives and no problems were found.

Here is what I HAVE found:

• The system usually hangs when there's increased disk activity, like when a browser is looking in its cache
• When the system "pauses" or "hangs", the hard drive activity light is on solid, but I hear no "scratching" from the hard drives, themselves.
• When the system "pauses" or "hangs", the DVD drive no longer responds and I can't open the drive tray when I press the button. I have to reboot the system to open the drive tray.
• With the DVD drive disconnected, the "pause" or "hang" still occurs
• The hang occurs even during a Windows "clean boot".
• The problem doesn't appear to happen when the system is booted from a Linux live CD (but I need to do more testing)
• The Windows Event log doesn't contain any entries indicating "errors" or "warnings" which might be related.

At this point, I'm about bit out of ideas of things to try. I'll see if I can use the system for a longer period of time with the Linux live CD, but using the live CD won't involve hard drive access.

This is starting to sound like a hardware issue, but I'm not sure how to properly verify that. Maybe I could disconnect the "data" drives (the hard drives other than the C: drive) and see how the system behaves.

Any other ideas?

Thanks!

Peace...