Powering down during boot-up

Having major issues with a recent build....long story short, CPU was overheating (not being actively overclocked) with stock cooler, so installed all in one water cooler - which worked like a charm for a day or two - now 9/10 won't progress past ASUS start screen - just powers down. Other times, can get into the BIOS and then load the OS. Have updated the BIOS, and set the CPU fan monitor to 'ignore' (as water pumps don't run as quickly as CPU fans and can flag a CPU fan error, which it has done). I've changed the mobo thinking it may have been an issue - I used an identical one and had the same problem. Maybe the CPU was fried? Any suggestions?

Mobo: ASUS MA97 EVO R2.0 AM3+
CPU: AMD FX 8150
RAM: 8Gb DDR3
PWR: Thermaltake 850W
GPU: GTX 960 Windforce
OS: Windows 7 64bit
Cooling: Deepcool Storm Maelstrom 120 liquid CPU cooler