Constant crashes in every game!?

Hey there, using an i5 quad core, 4 gigs of ram, P55A Fuzion MB, with (most importantly) a Radeon HD 6870 graphics care (1 gig).

Had no problems with this rig for the 2+ years I have owned it, ran everything perfectly (primarily a gaming machine).

Recently though when trying to run NWN 2 there was some graphical freezing (the graphics driver crashing, and me having to alt-tab back to windows to contiue). As first I thought it was just a problem of the game being old and not compatible with DX12, but the 'problem' has spread to other games also: namely Dark Souls 2 and League of Legends. Now any game I try and run that is more grapahically demanding than hearthstone freezes, goes to black and freezes my PC within seconds of loading up.

I've tried deleting the drivers and reinstalling them (I have the latest version), tried cleaning all the gunk out of my PC to keep the temp down (graphics card runs at a constant 70celc, CPU 45), none of it made any difference... recently when watching a stream on twitch I also had a similar crash

No idea what's going wrong, and I'm at the extent of my limited knowledge. Any help would be massivly appreciated.