[Solved] Help me troubleshoot a sporadically unresponsive system

Hey everyone,

I'm having some issues. Any help troubleshooting?

Symptoms:
+ Every so often my system will go unresponsive. Usually within 15 min from the previous time.
+ The HDD light will stay on completely for this, although I can't hear it "thinking"
+ The mouse will move, the start menu will open, but opening a folder or app will just produce the spinning circle thing.
-- I'm guessing anything that it cached in RAM is responding, but it can not access the hard drive.
+ After a few min, the hard drive light will flash, the disk drive will audibly "think" again, and the system jumps back into action.

What I've tried:
+ SMART monitoring says the drive is healthy
+ full system error check on bootup with WIN7 didn't find any problems (although it seemed to take longer than it should.)
+ Quick Virus scan = clean

I don't hear any warning signs of hdd failure. No error messages.

Thanks guys!

System:
Win 7 Build 7100 x64 Ultimate
core2duo e6400 2.13 @ 2.66 undervolted (stable)
4 GB Corsair DDR2 800
Gigabyte p965-s3 rev1
Powercolor AMD 4890
Corsair 750W PS
WD Raptor 74GB Sata (system drive)
Seagate 300 GB Sata
Samsung 750 GB Sata

I am running AVG Free 9. Like I said, I have already run a scan and came up with nothing. I let it go overnight, because it would pause while doing the scan too. It took a long time.