Hardcore Mystery, everything replaced in Gateway pc, still shuts down

Gateway desktop GT4016, shuts off after awhile, now fan & light turn on for 2 seconds, shuts down.
Replaced: Power supply, motherboard, on/off switch, CPU, bypassed hard drive, optical drive, & memory sticks, still shuts down.
After replacing CPU, it came right on, ran for about 8 hours, shut itself down, played with on/off, eventually got it to power up & work for
another 6 hours, shut down again, now will not turn on.
Before this, substitued line cord, & different outlet.
History on this is I would have to unplug from behind a couple times, maybe more, to get the power to come on.
The data light would come on, but not the blue light to connect to the hard drive.
I transplanted the hard drive into a similar Gateway, but not compatible, got a blue screen trying to "learn files" but kept going round & round.

Anyone think of something I missed?

 

According to your description it should be a completely new pc:-D

Video card,Switched that out:confused

Download and run speedfan and post your temperatures, if your temps are fine-

Download and run intel burn, set for maximum stress while checking temperatures in speedfan.

Download and run 3dmark or heaven benchmark or any 3d intensive benchmark.

Use only one stick of memory while testing, don't connect any extra drives.

Go to control panel, system,advanced, startup and recovery un check automatically restart.

 

Did you replace the cpu fan? If not, have you tested to see that it is spinning up?

 

I tested the fan, ok, so not a cooling problem, however the fan used to come on indiscriminatly, so am guessing something is not properly turning on the fan to the heat sink when it gets warm.
I ordered another pcu, as soon as I get it to turn on, am going to find a way to look up the operating temp.
What puzzles me, is a new motherboard, I assume there is something on that to automatically turn on the fan.
(as soon as I installed the last pcu, it powered up & stayed on for 2 days)
There is no video card in the black slot, so am assuming it is intregated in the motherboard.