If it did it only when gaming in an intense game, I might suspect the PSU is failing. Especially with dual GTX 770s. But if it does it even in MS Word, there may be other causes. Like the MB. First thing to check is that the 24 pin and 8 pin PSU cables are firmly in their headers. Make sure the finger clip is over the locking tab so they don't back out during warm up/cool down times.
Next, you might try removing one of the gfx cards and try it like that for awhile. If it still does it, switch cards. That will eliminate the card(s) being the problem.