Did you have the same cpu when you had problems with the 5850's ?
Tried a different power socket / another room ?
I'd try that, and a different kettle lead aswell, and a different surge protector.
Using any adaptors in there, ide to sata, fan cable adaptors, I'd drop the fan controller just incase something broke or is being overloaded and decided to draw too much power, causing the psu to reset or something.
Go into mobo bios and disable every last unessential onboard thing, sound, network, everything you can get away with.
Beyond that the CPU seems to be the last thing you haven't replaced so I'd consider that, I take it with watercooling you were overclocking before? It might just have lost a little stability, I'd try stock speeds with increased voltage, whatever you were using when overclocked maybe.
What kind of airflow do you have in the case normally with watercooling around the CPU, do you have enough cooling for the CPU vrm's, chipset heatsink?
I'd try dropping memory timings a little but not too much, try 2t command rate.
lastly software wise, I'd try a fresh install with minimal stuff added, and a 3d app, however I'd do all the windows update, download the direct x installer and make sure thats up to date aswell and any patches for game/benchmark to make it as up to date as possible.