This is a pretty new proggy from the F@H team. It uses OpenCL to test GPU along with video memory. Screw the rest, this thing is even better then OCCT. It can catch errors others wont, TRUST me.
I find it even catching errors after long gaming sessions of different game engines that werent crashing or artifacting.
Check it out
Leave it running overnight and if you see 0 errors then you should be good to go. If you see even 1 error then it means your OC isnt stable like you assumed
Only quirk is that it runs in cmd line. So no mousey.
Just shift+ right click the folder its in.
Then type memtestcl xxxx (being number of megs your video card has to be tested, ex 1gb=1024, 2gb=2048, etc...) space then yyy ( y being the number of passes you want the test done, i just put 1000
Trust me this thing works great. You guys need to make a sticky to it here on OC section.