Yup! babymac is very right. All my exact sentiments.
In fact, i ever got his incident stated in point 1). i.e. very
interestingly, its my Graphics card, not latches properly, so that
when it heats up, there is apparently some expansion due to heat,
and the connection comes loose. This then causes my processor to
restart,etc and surprisely when it cooled, i can start my computer
again. This can also occur for the PSU.
PSU, some possibility, like if you buy those lousy brand PSU which
gives unreliable wattage and can die off fast one. So after some
time of running like 1yr or so, PSU may become unstable, sometimes
when you start using some programs or rendering graphics, power may
not be enough or wat and cause autostart. 
