I had the exact same problem, I tried for 5 days to fix this problem, but my motherboard and CPU where only two month old, I checked the ram, one at a time, and the gfx card in another comp, even applied new thermal grease to the CPU and gfx card, I mean I really stripped down the CPU and heat sink and the gfx card right down to the bare boards and washed the heat sinks on both, making sure no water residue was left, I even tried applying one ram stick at a time, by now this was pissing me off, I even washed the CPU cooler to make sure there was no dust at ALL, I managed to re-flash my BIOS, even though it would reset in the BIOS, but managed to get a flash successful, I was so frustrated, after all I am 56 and been building pc's for 30 years, I stripped down the PC again and again.
Then just when I had given up and used my tablet for four days instead of my PC, and asked my beautiful partner to purchase a power supply for me as well as other part, I was willing to buy the most expensive UPS, I decided to search the net again for problem solving, I knew I had purchased all the best parts for my PC when I built it just a couple of months ago, and just hours before spending a shit load of cash to have my PC working again, I read on the net that a load of guys where having this same problem, then some one suggested that I eject the cmos battery and clean it and replace it immediately, I though you got to be bloody kidding, remember I have been building PC's for a very long time, never in the past have I ever had to clean the battery on a motherboard that was only two month old. MB and CPU where purchased at the same time, the bastards that sold me the mother board, had used it in the shop, god knows how long, anyway, I ejected the battery wiped it with my dirty T-shirt, replaced it, and hallelujah, it revived my PC, it is now running better than ever for the last couple of days, my PC is as follows:
CPU 4790k, still cant overclock the bloody thing.
gfx card amd HD6950, yea I know old but plays everything, overclocked.
Mem G-skill Sniper 16 gig
HDD 10TB, yes that's a lot as I am a designer
CPU cooler Thermaltake frio, excellent cooler
Power supply thermaltake toughpower 775w
motherboard MSI gaming 7 Z97
case Antec dark knight DF-85, seven fans for god sake, plus extra fans to keep gfx card cool.
I am so happy, my comp is ok, but hell I have spent hours and hours, no wait, it was days and days, of frustration, and to think all it was, just a bloody battery that needed cleaning with a bleeding dirty T-Shirt hahahahahahah
Now I am a happy chappy once again, just goes to show, never over look the simplest of things.