if your ram is running 1600mhz then its set up correctly it will be running at 3200mhz (ddr =double data rate).
i'd say next thing to try is another gpu, ask your friends if they'd be kind enough to lend you one for testing, if not head on to the b-grade section of overclockers and pick up a card, £100 mark should work for testing, don't go all out and buy a new one as the problem could be elsewhere.
i'd say its something between the gpu or the psu, but hwmoniter showed excellent voltage readings for your psu so i recon its your card, but testing another is the only way to be sure.
but going back running all the stress tests should have showed problems as they are the worst case for a pc, and all passed.
The games your having problems with, did they get any updates lately? or are they in alpha phase (pre release?) could be a update has knackered the game or the alpha release has got bugs in it unbeknown to the developer