I have done all as mentioned above and no change, I've had 3 random turn offs today. I do agree that heat is an issue but not all the time. I've encountered many times before when the pc will crash/instantly shut down when the pc has just been booted and is cool and low temperature.The following checks require the computer case to be opened so take the following safety precautions, 1st, disconnect the power cord from the wall socket, press the power on button to get rid of any residual charge in the system, remove the side of the case, take anti static precautions before touching anything inside, you can do this by touching a bare metal part of the case.
Inspect the CPU cooling fan and heatsink, make sure that it is securely mounted atop of the CPU and that the cooling fan and heatsink are not clogged with dust.
Remove the video card, blow out the slot and re-seat the card securely, do the same with the memory sticks, clean all internal fans, if you use canned compressed air do not let any fan spin freely as this can damage them, use something like a clean lollipop stick to hold the fan still.
Ensure that all internal connections are secure and that there are no loose components, write down the PSU details (make and model) re-assemble the computer, boot up and see how it goes.
Just a cautionary note - if you do use canned compressed air do not upend the can as it is possible for fluid to be emitted and do not use a hoover or cleaning out dust, they create static that can damage the hardware.