just got the GA-Z77X-UD5H and having the same issue. Some cold starts and some restarts I get the "Boot Failure Detected" bios halt at post. Seems to happen randomly but is about 40% of the time at stock settings, Seems to happen about 80% of the time with any slight overclocking or even just...
Well ... technically its always 12V no mater what, but in series the two fans would divide the load evenly and spin at a lower speed. The resistance would double, making the fans spin slower as if they were only getting half the voltage. Ohms Law
I've never had a problem either selling lapped cpu's, if anything they were more desirable.
The bigger heatsink and paste should get your temps down some. Lap the cpu and heatsink and get them perfectly flat and polished. Nice even cote of AS5, but not too much. I would think with everything set...