It might help to know with what settings you messed with and which you left alone.
Maybe a list of parts and settings would help to start with
First part I would suspect would be the power supply.
Of course I am somewhat of a overkill power supply fanatic..too much wattage is never enough IMO
My system has a 700 watt power supply only because I could not afford a 1kw power supply when I built it.
This only happens rarely? or is it becoming more frequent?
You might need to reseat everything if you haven't already done it..e.g. pull the ram and put it back same with the vid card/s etc, and while you have the parts out, inspect them for defects, and blown caps and such.
You might want to inspect the case to see if there might be any metal shards sticking towards the mobo and possibly causing an arc (I know it's rare but it does happen)
A friend of mine had re RMA a power sup because he could just push on the side of the supplies case and the computer would reboot or just shut down. I wish I could remember the brand..the guts were loose inside and shorted out with the slightest bump....not good for a brand new build...good thing he noticed before it took down his expensive new parts.