8GB RAM problems on G33 MB from Gateway FX4710

Hey all,
I recently bought a refurbished Gateway FX4710 for a song, and I've been working on it to make it into a cheap, stable rig for casual gaming. I've taken care of one of its weak points by replacing its stock 400W PSU with a OCZ Fatality 550W. Another weak point is its micro ATX G33 motherboard, which is a Gateway proprietary board and does not allow changing any voltage or speed settings on its bios. I've tried to see if bios drivers from the board's apparent manufacturer, Foxconn, would work, but so far have had no luck. Also, there is only 1 bios update at the Gateway site, and it's actually just the same driver that the computer came with

This relates to my current problem, which is with 4 sticks of 2GB RAM I just got. They are two 2x2 GB of DDR2-800 mhz RAM from OCZ and Patriot that I bought separately to take advantage of rebates, installed with appropriate pairs (http://www.newegg.com/Product/Product.aspx?Item=N82E16820227267 and http://www.newegg.com/Product/Prod [...] 20220335). Both are rated at 4-4-4 timings at 2.1v. They replaced 2x2 GB 667 mhz RAM, which ran at 1.8v.

Since installing them two days ago I have been able to boot and run programs fine on my Vista 64, but with 3 or 4 scary crashes in between. They have occurred once in the middle of running programs, and a few times right after I lightly bumped or vibrated the case - by shifting it slightly on a table, or simply getting up and walking around. What happens each time is that the screen freezes and says "no input signal" after a few seconds, and does not turn on again when I reboot. The first two times, I removed and reinserted the video card and RAM, and the computer was able to boot fine afterward. The later, two crashes I found I could just remove and reinsert the RAM sticks to "fix" the issue, and then the PC would go back to hunky dory.

Interestingly, the memory passed all system memory tests, and I ran multiple memtest programs overnight with 0 errors found. On CPU-Z or SIW I see that the 8 GB are currently running at 800 mhz at 5-5-5-18 timings.

So I'm completely confused by what is causing these crashes. I hope to know soon so I can decide whether to return the RAM, which I would have to do soon to avoid missing mail-in rebates. I'm guessing it could be:

1. Bad fitting RAM - either a fault on the RAM sticks or the MB's slots causing them to pop out slightly when the case moves?
2. RAM voltage issues - does high-speed RAM try to use more than 1.8v in a slot, and hence possibly causes crashes?
3. Something else - insufficient PSU power still, software, etc.

Thanks!

Specs:
Gateway FX4710
Q9300 C2Q @ 2.5 ghz
nVidia 9800GT
2 x 2 GB OCZ Reaper DDR2 800
2 x 2 GB Patriot Viper DDR2 800
640 GB HD, DVD-ROM
Nothing in PCI or PCI-e x1 slots.
G33-type Gateway motherboard (g33m05g1)