Trying to resolve random lockups

Hello,
I've been working with my most recent build over the past month, and everything's been working great except for the fact that every once in a while it'll lock up on me. This is mostly during games, but that might be because I've spent most of my time on it in games, as its frozen a few times while I just have a firefox window open or word or something. This is at stock speed and voltages, nothing is overclocked or anything. The system completely locks up: the screen freezes, num lock is stuck, the last ~.25 seconds of sound loops, and sometimes it restarts after 10 or 15 seconds. Other times I have to manually reset it. The only related event log I see in Vista is "the previous shutdown was unexpected"-not very helpful.

My system is as follows:
CPU: Intel Core2Quad Q9650 (3.0 ghz @ 1.1V)
MB: Asus Rampage Formula motherboard
RAM: 4x2gb OCZ DDR2 1066 RAM
Video: Sapphire HD4870x2
Sound: X-fi Titanium PCI-E
PS: PC Power & Cooling 750W Silencer
HD: 3x640 gb western digital caviar
2x LG DVD-/+R burner
LG Blue-ray burner
D-Link Wireless-G PCI adapter
Antec 1200 case
Default BIOS settings
Vista home premium 64-bit

So far this is what I've tried:
Memtest86+: no errors after several passes, about 4 hours of running
Prime95:Stable after 8 hours
Prime95+3dMark06:stable after 6 hours
Removed 3 of 4 DIMMs (I've tried 2 of the 4 memory sticks)
Uninstalled, cleaned, reinstalled ATI catalyst 8.10 drivers
Uninstalled/cleaned creative sound drivers
Removed sound card
Updated Chipset drivers

After all this it still freezes.   It seems completely random, sometimes it freezes after a few minutes, sometimes it freezes after a few hours, and on good days it works fine
What have I not covered? I was thinking of running diagnostics on the hard drives next and trying different RAM slots or the other 2 sticks I haven't tested alone. Or maybe there is a power cable or something loose somewhere, so I could redo all the cabling.

It could be overheating, but temps look ok, cpu core temps are <30c at idle, around 48-50c after 6 hours of prime95, gpu temps peak at around 80c which is pretty high but I think normal for this card. Nothing sticks out as being out of normal ranges. The longer Prime and 3dmark runs were done overnight though, so they might not completely represent normal conditions as the ambient temp was probably significantly lower than during the day. I think I have 9 fans in my case though, so it shouldn't be an issue. Maybe I'll run it with the cover off just to be sure.

It's never frozen on me in safe mode, or before I install the video drivers, but I haven't spent too much time with it. Still, it makes me kind of suspicious of the video card. I don't have an extra video card to try at the moment, but I was thinking maybe I would just RMA it and see if switching the card out would fix it. I'm kind of confused though, because it went through a few hours of 3dmark just fine. Its frozen on me many times while playing Oblivion, CoD4, and Half-life 2 (those are the only ones I've been playing recently), and also while just browsing in firefox.

The past few days its been getting more frequent, to where it might freeze a few times in a day.
Any thoughts would be greatly appreciated.