If you didn't upgrade the P/S from that setup then its possible you are giving it enough juice. Its also possible that you have a heating issue. As shyguy asked is it not working only with games, or not working for the over all PC?
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The problem as a bit too vaguely stated in my post above seems to have gone away/doesn't really matter, as after ten or so attempts it boots up normally.
The problem I 'was' getting was either the screen didn't register the computer turning on/ the screen would turn on and the sony etc logo would pop up but then it would freeze.