Are your RAM timings set correctly? On your RAM it will give you a frequency, most likely 1600mhz and some CAS latencies, e.g. 9-11-11-30. Check what your values are and then go into the BIOS and check what they're set to there, the same with RAM voltage, most likely around 1.5v. In your BIOS, check that either all the settings are on "AUTO" or set them to their correct values. Incorrect values there can cause stuttering.

I tried to do that some time ago but I couldn't access my BIOS through Win8.

So far as I know, W8 uses hibernation by default instead of shutting down, make sure it's shut down completely and it should be fine. Shutting down completely might require changing an option in the control panel or something though.

Yeah, I checked that and the BIOS option wasn't in the startup menu. I can't think of a reason it would be like that though, considering I've never messed with it before.

Tried something like http://www.makeuseof.com/tag/how-to-access-the-bios-on-a-windows-8-computer/ ?

Yeah, that's the tut i used.

I don't really know what else then to be honest, short of finding a way to get into BIOS. You could always completely shut down (either through W8 if you can, or something like turning the PSU off directly) and unplug your drive so it doesn't boot then you should be able to get into the BIOS.

You can use Speccy to find your RAM timings I think.

http://puu.sh/650Dm.png

And it'll also tell you the temps (though not as reliable as other programs I'm sure)

http://puu.sh/651iv.png

I checked the CAS Latency of the RAM I purchased afterwards to upgrade my system (http://www.newegg.com/Product/Product.aspx?Item=N82E16820239317&Tpk=KVR16N11%2f2&IsVirtualParent=1) and it only lists the CAS as 11. Don't really know what to make of that.

Also to note is that every time I did something to this computer (changed the OS, changed the OS back, blanked the HDD, replaced the motherboard, and replaced the PSU) my computer's performance degraded.

It's only a year old too.

Blanking your HDD doesn't help if the drive is dying, loads of delete and write cycles esp when reinstalling everything will make it age a lot faster

Run ur test with this: http://www.hdtune.com/
Select the free version

That's the one I used. Everything checked out.

Mind posting screenshots of the speed test and error test?

I don't have them, I did it a while ago after I blanked the HDD (hence why I can't use the free version anymore)

edit: apparently I can now, I'll re-test it and get you those screens

http://puu.sh/6532w.png - Speedtest (puush wouldn't work for the screencap so I had to grab the whole desktop, sorry)

SMART data is coming up fine. Doing error test now.

http://puu.sh/657FG.png - Error test. Again, everything checks out

huh my version doesn't expire, maybe try deleting and redownloading a newer version

edit: I see..

Sounds like it only leaves your ram to be the issue, test it with this
http://www.memtest.org

Tested the RAM as well with memtest (forgot to mention that in OP). Again, as usual, nothing.

That is really strange. Mine had the same problem awhile ago (last January). I left memtest on to do multiple passes and check for extensive errors and sure enough my RAM was corrupt and causing my computer to do everything you described. It's strange to me that even though you have the same symptoms, the memtest is coming out fine. Is your PC custom built? If so, is everything securely locked down?

Nope, prebuilt. I still don't know about those ram timers, though, considering I can't find the timings for the 2GB stick I have installed (there's a 4GB stick that came with the machine and a 2GB stick that I later installed, from different manufacturers).

Try booting up with either ram stick (only one, first with the old one then with the new one), the way you are upgrading your parts is very messy and does not seem to be composed very well

Yeah, it's kind of messy considering the only thing I wanted to upgrade was the video card and the psu before things got out of hand. I'll do that.

It should say on the stick of ram itself what the timings are

edit: looking at that speccy picture, the timings seem off but speccy has been known to be inaccurate about things, check CPUz and go to the Memory tab

http://puu.sh/65i19.png

Again, can't grab windows for some reason

look for 'xmp' in the bios and load the profile - this will give you the correct ram timings easily
it would also help to list your pc specs (screenshot speccy) to give us a better idea of what the problem could be

http://puu.sh/65j33.png
as

WRONG ONE

So your RAM frequency is set to 1066Mhz and your timings are set to 7-7-7-20 which is not CL11, although I can't quite find what the CL11 timings should be. Either way, get into the BIOS somehow and either load the XMP profile, if the frequency changes to 1600 it should be right, or manually set the timings and frequency to "auto".

I don't even know what's more unsettling about this picture, dual channel ram with two differently sized DIMMs or the lack of a northbridge.

Have you already tried booting with only 1 stick?

Could we get screenshots of the "SPD" tab (all slots preferably).

http://puu.sh/65KuR.png
http://puu.sh/65Kw8.png

I managed to get into bios but there's no XMP profile to load. Everything is on auto except the ram voltage which is set to 1.650V.

I'm in the asrock bios btw