New computer crashes

Hi guys, Two weeks ago I took the plunge and build my first system. Despite a few hiccups, I had the machine running well until about 3 days ago, when it started crashing arbitrarily while watching video streamed from various websites (ustream, livestream, youtube). What happens is the video and sound freeze, and sometimes I'll get a brief BSOD before it reboots or shuts down. As far as I can tell, it shouldn't be an overheating issue. I've using an antec 900 and aftermarket cooler, but this still happens even though the computer is at all stock speeds and not running anything particularly intensive. The processor is an AMD Phenom II x4 955, gpu is HIS radeon HD5850, ram is OCZ AMD BE 2x2, and the motherboard is ASUS M4A890GTD PRO/USB3, with Windows 7. Is there any type of diagnostic tool I can run to pinpoint where the issue is coming from? I've read various forums and topics, but there's so much information that I don't know where to start, and am quite frankly frustrated by the whole thing. Thanks for any help.

How do you know heat is not the problem? What are your temps? Did you apply a proper layer of TIM?

and aftermarket cooler

That concerns me. Is your AMD CPU a retail version? That is, did it come with an AMD supplied heatsink fan (HSF) assembly? Understand that according to AMD (and Intel has the same policy), the use of any cooling solution besides the one provided by AMD voids the warranty as clearly stated in the AMD CPU Warranty (their bold):

This Limited Warranty shall be null and void if the AMD microprocessor which is the subject of this Limited Warranty is used with any heatsink/fan other than the one provided herewith.

The good news is since both AMD and Intel warranty their boxed CPUs for three years, and since replacing them at their cost is not something they want to do, both make excellent cooling solutions both in terms of cooling abilities, but also in noise levels.

How do you know heat is not the problem? What are your temps? Did you apply a proper layer of TIM?

and aftermarket cooler

That concerns me. Is your AMD CPU a retail version? That is, did it come with an AMD supplied heatsink fan (HSF) assembly? Understand that according to AMD (and Intel has the same policy), the use of any cooling solution besides the one provided by AMD voids the warranty as clearly stated in the AMD CPU Warranty (their bold):

This Limited Warranty shall be null and void if the AMD microprocessor which is the subject of this Limited Warranty is used with any heatsink/fan other than the one provided herewith.

The good news is since both AMD and Intel warranty their boxed CPUs for three years, and since replacing them at their cost is not something they want to do, both make excellent cooling solutions both in terms of cooling abilities, but also in noise levels.

Well I've been monitoring the heat levels with AMD overdrive, and it has never gone above 36 degrees.

My packaged HSF got damaged by a clumsy friend, so I ordered an Arctic Cooling Freezer 64, and applied Arctic Cooling thermal compound.

36°C is great, but I would verify with the monitoring program that came with your motherboard - I think it is called ASUS Probe.

You also need to check your graphics card temps. What PSU are you using? Unfortunately, there is no one program that tests all. You can test RAM using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Allow the diagnostics to run for several passes or even overnight. You should have no reported errors.

Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
or
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.

Alternatively, you could install a single RAM module and try running with that to see if it fails. Repeat process with remaining modules, hopefully identifying the bad stick through a process of elimination.