Had the same exact problem checked and re-checked everything. I did send the mobo (gigabyte z77-d3h) back but the replacement was the same. A different psu had the same results, took out cmos battery, used 1 stick of ram everything had my computer expert uncle have a look and still the same and as I had already had a replacement mobo it must be the cpu, so sent that back and awaiting a new one next week. One thing which was strange was the stock cpu fan not spinning upon start up but an after-market cooler did.