Wow nice rig,
Bit of an overkill for Ubuntu aha. I very much doubt it is a hardware problem (unless its your GPU) and more a software issue. If you open up the CPU control centre (its been a while since i have studied a linux distribution [studying computer science at 6th form]) if you have a look how much CPU usage is going on and what linux is "swapping" (swap space) as the way linux handles high amounts of traffic is different to windows (more efficient but can have flaws). I doubt swap space is going to have an effect with all your RAM I must admit im rather jealous!
If you can find a way to look at the CPU that could be an entry point.
What program are you using when it goes gray? In ubuntu a gray screen means its hung, if its from the multiverse rather universe it could be bad code.
If you post any answers to this hopefully we can find an answer :-)
Quick edit: If you have a look at the CPU usage when it goes gray, i have a theory.. May not be correct as it could be a number of things but if the program is written badly and becomes intractable it may be using all of your cpu (i7 unlikely but still i guess small chance).
Also what graphics card you using?
Edited by bian101, 21 March 2012 - 12:13 PM.