Computer keeps turning on and off

Hi i bought this gaming computer for my son and the first day we had it it worked good but kept turning off by itself, so the man that built it said i need a new power cord he put that in, we went home and it started again going off by itself. But this time it wouldnt come on anymore, took it back to him and he said i needed a power supply that only worked for the computer got that, he also said he had to replace the mothorboard due to us not having this plugged in by itself on the power supply. Got the new powercord worked for a couple of daysand now it is doing the same thing again going on and off and now it wont come again at all. I now nothing of computers but if I pay for a another mothorboard what will stop this from happening again. Anybody have any ideas please help. Thanks