Extreme Gtx 780 issues

lately i have been having issues with my GTX 780's overclocks, i had flashed a Custom VBios an EVGA 780 FTW bios and i pushed the OC to 1.25ghz and 7.2 on the memory and it seemed completely stable i was doing 71C on full load but after running a benchmark and come near that clock now, my PC screen becomes blank and turns off due to no signal but the PC remains on and does not do anything unless i force a reboot, does anyone know what the issue is? 

You pushed the card too hard. Scale back the oc. 1.25ghz core clock and 7.2 on mem is bloody high and could have very well f*cked the card.

If the card is now faulty - better hope you can flash the original bios back - the rma refusal may hurt.

Just because temps seemed ok doesnt mean its all good to push a card to past it limits unless you are ok with perhaps killing it. Silicon has its limits. 

Best of luck matey.

No I mean it works stable till I start putting a heavy load on it like fire strike, everything in my system is heavily overclocked and I only have a 700w psu , do you think its the power supply that is holding back the card because i mean with gpu boost 2.0 enable my card has auto pushed itself to 1302 mhz and then the system crashes as of then and theoretically can hit 1.4ghz (by the stats shown in unigine)  I have so far slowly rolled off the 4.6ghz to 4.2ghz on my cpu and the gpu seems to be more stable, so i imagine ts is the psu if that sounds right? (forgive the grammar mobile use is difficult)

stable until heavy load does not equal stable.

as for psu - depends on the unit - make/model? Remove the cpu overclock and see if firestrike is stable.