under load its either the heat of your system (the system temp console can show you all of this) or a failing componant (short, RAM, something not plugged in a socket correctly) if it boots and runs then chances are that the PSU is fine (depends how it sounds when in use along with temp). with the limited infdo avaliable, i would say take it all apart (what you are comfortable doing) and chech everything is seated correctly and ensure that air is getting to places that need it.