Display crashes when running demanding games and apps, GPU or PSU?

hi my home built machine has been running like a dream with no problems at all until two weeks ago when I started up there was no display I swapped the video card from the xfx amd hd 6870 to a new asus amd hd 6950 aand updated the drivers with amd catylist software.
all was ok for a week or so then I had been playing skyrim and installing mods I had problems installing F.O.S.E so I abandoned it and went to play the game the splash screens went as normal but as soon as my save started to load my screen went black the audio carried on for slightly longer before freezing, my machine was still powered up the keyboard and mouse were lighted up as were the led's on the motherboard chassis etc.I had to hold the power button and reboot all seemed ok and I tried running different programs and games to attempt to ascertain whether it was a one off crash or a hardware problem and I experienced the exact same crash under more or less the same circumstances. although less demanding games will run without problems.
I have also noticed some small changes with my system for instance smaall graaphics glitches and a general sluggishness.
I googled extensively and tried checking the hdds scanning for viruses even a clean install to no avail I@m stumped guys, I'm checking the event viewer but I'm a bit out of my depth I think so before I go replacing the psu. please enlighten me with your encyclopeadic knowledgebase thanks.p.s. I am the only user on this computer.
sorry if this is posted in the wrong place its my first one