Computer shutting off randomly

Hello all,
Hopefully I'm in the right forum.  I purchased my rig from DS about a year and a half ago and up until 2mos. ago it ran flawlessly.  I purchased this computer to play sc2 and wow on full settings.  And it worked great, no issues.  I stopped playing both games about 6-7mos ago.  And about 7 weeks ago I downloaded Steam and FFxi and started playing that regularly.  It was also about 7 weeks ago my computer started acting up and just randomly shutting down.  At first it was just when I would do teleports in ffxi, and it was once, maybe twice a week.  Then it would happen while on the internet, for example at trailers.apple.com watching movie trailers, or any website where ads were being played along the side of a browser page.  It now happens at least once a day sometimes more.  My computer would just shut down.  I can almost guarantee it will shut down if i do a teleport in FFxi, like 90% of the time.  Now the shut down, my monitor just says "No signal" and my computer becomes quieter, but the fans continue to run and the led lights all remain lit.  However, if i hit the power button nothing happens, reset, nothing happens, hold the power button for 10 seconds, nothing happens.  The fans just run and the led lights stay lit.  the keyboard and mouse both shut down as well, they are both wireless logitech equipment and all lights shut off.  Only thing I can do is unplug the tower, and hold the power button for 10seconds.  Then I can plug it back in and power it back up.  Just flicking the switch on the psu does nothing.  I've checked the power supply and it seems fine, not dirty, clean, from what i can tell all capacitors look fine, none bulging or leaking/cracked or anything like that.  My guess is it could be the psu, but I've read graphics card, mobo, etc from Google searches.  Last week I ran furmark on my Radeon HD 6870 1gb card on benchmark burnin 1920x1080 15min and it passed.  I went into catalyst control, turned off the AMD overdrive and ran the furmark again.  The moment it started, the screen flickered, then shut off.  So I turned overdrive back on, ran Furmark, and it passed again.  Not sure what that means.  Ran Prime95 last week also for about 25min. nothing happened, ran fine.  However i only ran it for 25min.  Event logs show nothing.  kernel power 41 critical.  And that could just be from me unplugging my computer.  Ran virus scan, ccleaner, malwarebytes.  Nothing came up.  Any help would be appreciated, here's my specs

Gigabyte GA-890GPA-UD3H
AMD Phenom II x4 970 3.5 overclocked to 4.0
750w Digital storm certified power supply
4gb ddr3 1600mhz digital storm certified ram
500gb hdd
Radeon HF 6870 1gb
23" Acer H233H monitor

I'm sure I'm missing something, if i think about it I'll add it.  Thanks for reading all.