WOW i've never seen such a bad (supposedly) high wattage PSU! That must be a "No1 bottom" place holder! Seems like the OP is lucky his PSU hasnt exploded yet when the GPU tries to draw more power. It is actually amazing to see how modern PC components (mobo, gpu etc) can sustain alot of beating from a low quality PSU, since the OP mentions he has this shutdown problem "for a while now". A a corsair CX500 500W psu will to the job for you, or actually any 500W+ PSU with at least 80+Bronze rating from a known company.