Trouble shooting

So i bought this pc and it was over heating and shutting off so infigured it was the thermal paste so i boughr some and added it to my cpu mating surface then put it all together and booted the pc up and it runs longer now (about 3 or 4 minutes) then shuts off all the reserch i see says it might be a power supply so im about to order one but would like to know what you guys think