Help please

so my problem started a few months back, when my PC would freeze up when watching videos on youtube, specifically youtube, i later learned that the problem seemed to have a connection to the resolution i was watching youtube videos in, anything over 360p ends in a freeze. fullscreening the video seemed to make the freeze occur faster

any when i say freeze i mean complete freeze, totally unresponsive to everything, mouse stops moving, sound stops playing, everything stops.

but if i do something else and just leave my PC running or perhaps play a game for a few hours and then watch a video, it worked fine. the problem only occurred when watching videos.

this problem was rather easy to ignore, i would just not watch videos for a few hours after booting the PC.

two weeks ago, i would sometimes get a random black flash, and a note would pop up saying that my screen driver stopped working for a second and that they have restored it.

the real pain started a few days ago, when all of a sudden it would start freezing without me watching a video, just editing a word document or looking at reddit, pictures or gifs.

a day or two after that it completely stopped being able to boot.
there was no light or life in the mouse, keyboard or headset.
the screen was displaying the text "no signal" which is the same message as when it is not plugged into anything. i open the side and check it out, everything is lit up and all the fans are on.
so i pull out the PC and open it up, first of all i deduce from the previous issues, that this might be because my video card is dead.

now this is strange because the video card is relatively new (less than one year old)

so i pull it out and replace it with my old card.
boot up the PC and it worked again, great! maybe a faulty video card?

BUT

it now has the same issues that the other card had before it started dying.
and it was completely fine when i replaced it with my newer card. i merely needed an upgrade, so i found a beefier card at the time.
this strikes me as weird, because it was fine before i replaced it and it has not been used by anyone since then.

today the issue has really hit the fan, when it only takes 2 minutes for it to freeze up and it does not matter what i am doing at the time.

BUT it seems that starting up a game, makes it unable to freeze, as i have been inside wow for the past 20 minutes with no freeze.
really weird.

doing anything but playing a game or anything that requires many resources makes it freeze.

-NOTES-
before you even ask, yes i frequently update my drivers and have done so MANY times since i started having these issues.
although a few days before the new card crashed, it gave me an error when i tried to update its driver.
i also clean the PC often, so it is mostly dust free.
there is no slow down whatsoever, turning it on and off when it freezes does not affect performance in the slightest.

i guess i should post my setup:
Graphics card:
NEW: Radeon R9 280x
OLD: Radeon 6870 1TB memory
CPU: AMD Phenom II X4 840 3.2 GHz
RAM: 2x4GB 1833Mhz
power supply: 500Watt
operating system: windows 7 64bit
Hard drives: 1 SSD with my OS on it and a 3TB hard drive for everything else.
Motherboard: ASRock 870 Extreme3