You don't mention what PSU you have, but it might not be up to the task of running your system. I would recommend a name-brand 750 Watt PSU with a gold or platinum rating as a minimum. Power spikes are where the cheap PSUs fail, not being able to maintain the voltage levels during heavy loads. That causes things like CPUs and GPUs to crash. So first of all, tell us about all the components you have installed.
Second, try measuring CPU and GPU temperatures while idle and then when gaming, using the AMD Adrenalin software running in the background. If you see the temperatures approaching more than 80 C, then you are probably crashing due to excessive heat levels. You might not have adequate air flow through your case, or you are possibly over-clocking something. But you do say you are trying the under-volting approach and it still happens; I still suspect the PSU.
As Albert Einstein said, "I could have done so much more with a Big Al's Computer!".
