Computer lockup during gaming

Hi everyone. So I've had this problem for quite a while now. Tried quite a few different things but mainly just living with it at this point. Getting this out there to see if anyone might have some ideas as to what the culprit is.

PC specs:
Intel Core Duo 2.7ghz
3gig ram
Windows XP (with sp3)
Nvidia Geforce 9600GT

PC was custom built myself a few years ago. At the time I made sure the power supply was sufficient to run everything well. (But i'm not convinced that its not the problem)

On to the Issue. It only happens while playing a game.

At seemingly random times, my textures in game will go bonkers, sometimes buildings will turn bright white and other strange translucent colors, sometimes weird textures will appear randomly everywhere. such as a 2D image of a building or character just sitting on the ground somewhere, replacing the ground texture. It's quite a trippy experience to say the least.

This problem I can remedy everytime by opening the video settings and changing my texture quality. I usually go back and forth between low and medium as the problem presents itself.

While this is slightly annoying the real problem is that randomly (sometimes once every two days and sometimes twice in twenty minutes) my computer will freeze completely and can only be fixed with a reboot. This occurs differently depending on the game and other unkown factors. Sometimes I'm able to remain on skype with my friend and talk to him about how stupid it is that my PC just locked up. While other times everything locks up and I'm unable to speak/hear in skype and instead get a wonderful buzzing tone in my ear.

As for the things I've already tried. I have ran a ridiculous amount of virus/spyware scans and have cleaned everything I could find multiple times.

I have checked my GPU and CPU in and out of a game and they have never come close to overheating. I've even Underclocked my GPU a little just to see and saw no difference.

I have also updated my video cards drivers multiple times and even tried some custom drivers from different third parties with 0 improvement.

I have reseated every component in my PC multiple times with no results.

I recently ran a test on my GPU with furmark. It ran at 99% GPU usage for 30 minutes and the temp leveled out at around 80C with no abnormalities.

Currently I suspect that it's either the Graphics card / Power Supply / or a faulty driver somewhere that won't uninstall completely.

Another problem which I think could be related is that whenever I move my computer and/or clean it it refuses to boot back up correctly when I hook everything back up. The lights and fans will come on but I don't hear the HD spin and the monitor will remain black and without a signal. The only way I've found to fix this is to unplug the power to the HD, boot the pc, and immediately plug the HD back in. Of course I get a warning, go into the bios, hit save and it boots fine.

This only occurs once after I open the case. all subsequent boots go perfectly.
 
I have basically the same graphics card, and I have just installed the latest driver over the older ones over and over through the years with no trouble. I will be retiring it next month for a better more up-to-date card. It has served me well :) The stalls could be from a dying hard drive too
 
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