What do you mean by working on PC for 8hrs? As in taking it a part or sitting at desk and working on it?
Do all standard checklist items. Remove memory, try different combos with memory, if you recently took cpu out or install cooling solution, check the pins in socket and make sure cooler isnt screwed down too tight. Remove all DVD, HDD, SSD ect from system. USB headers ect. Only have what is needed. Also if you have another PSU sitting around, try it.