-GSOD/buzzing with 3d video/gaming

I have a dell 435 t that has been plagued with problems since I got it. I have had to have the motherboard, the hard drive, and monitor all replaced (though there was never really a problem with the monitor as it was the GPU that was failing.) I got extremely fed up waiting on the phone for tech support and gave up trying to get the GPU replaced. At any rate, when it works it works perfectly. However, it does crash around once a day.

I have made some changes/upgrades:

Vista-->win 7
Added an SSD
and now most recently I got a graphics card

In July, RIGHT AFTER my warranty expired with dell my 4850 HD died.

I bought a 7750 HD XFX and was quite happy for about a month. Made sure I driver-sweeped the old drivers and kept the catalyst/drivers up to date and have been left with a 95% EXCELLENT quality of video AND 5% GREY SCREEN OF DEATH/BUZZING problem.

It also has problems switching audio when I switch from my monitor to flatscreen with HDMI. It occasionally detects the audio easily when i hit "windows key" + p and occasionally will not. I recently got an error saying something to the effect that my HDMI output wasn't working correctly and haven't been able to reproduce that error. However, I did sweep my Realtek audio drivers, installed new ones, updated my chipset (which I just realized needed updating tonight), and I still got a GSOD. I have pictures and video, but am unsure how to upload easily to this forum.

My GPU passes all stress tests (Furmark, etc.), all dell PC dr. hardware tests, and I have the Ultimate Boot Cd and tried running some memory tests... no problems so far...

Like I said for the most part I am very happy with my machine, except the GSOD which rears its head about 1/3 of the time I watch a movie using VLC, WMP or Netflix/Hulu or whatever online flash based player. It does not necessarily happen when the GPU heats up or works for a while.

It ALWAYS freezes when I do certain things like walk in areas with high smoke/fog rendering in Skyrim. It ALWAYS seems to involve audio and video.

Any and all help would be *GREATLY appreciated*!

Thanks-- I'll try to provide as much info as I can if I left anything important out...