To check if the cpu is getting proper cooling you could enter the bios by pressing anywhere from delete to any of the function keys when the first screen show up, the main page should show specs like cpu speed and temperature, if its something like 80 degrees celcius then the fan isn't doing its job if its like 30c then thats not the problem. After check for loose wires preferably unplug everything and replug everything just to be sure, heck if you could rebuild the whole thing it would be even better, it isn't that complicated. Look for any signs of physical damage to any of the components that smallest sratch could mean replacement other than that its hard to help, good luck.