Specs:
Intel Core 2 Duo E8400 3Ghz (not overclocked)
Sapphire HD 7770 Ghz OC edition [New]
Corsair VS450 PSU [4 months Old]
2 x 4gb ddr3 ram
Gigabyte GA G41m combo mobo [Replaced recently with warranty]

Well i narrowed down my fault find in my system, the issue was reboots during Ingame / stress testing GPU, and during stress testing GPU + CPU. Checked all components and Im guessing its either GPU or PSU, so i need u guys to help me in finalizing in whats faulty..
So what i found out was :
1. when i tried my old GPU with overclocked core clock 690 mem 653, i DIDNT get any reboot during stress testing GPU/ GPU + CPU.
2. when i tried to stress test my new gpu with stock clocks core 1000 mem 1250, i got reboots during GPU stress test after a few seconds/minutes and direct reboot when tried to stress test GPU + CPU
3. So i underclocked my Hd 7770 to a random 800-900 core and 900-1000 memory and it still rebooted during the tests but sometimes directly and sometimes randomly
4. Next i tried to underclock Hd 7770 to core 690 and mem 653( as my old HD5450 clocks) along with -8% POWER control. With these clocks i didnt get reboots during the stress tests of GPU / GPU + CPU ,they went for a long time without any reboots...
5. BUT with my HD 5450 on these clocks Assassins creed 4 lagged like *censored* (Below 20 fps always) and i couldnt play on max Resolution whereas here with new GPU with underclocked clocks 690 and 653 I'm getting almost same FPS as stock clocks of hd7770. its Playable with like 30-60fps, cant make out the difference between these clock changes,maybe a little fps drop but THATS still a million times better than my hd 5450. Ingame with these lowered clocks on hd7770 i think GPU is 80%+ load most of the timebut I dont remember the load on GPU during stock clocks (i think lower) and cant find out too..due to reboots
6.Temps are always fine ranging from 30-60 for both CPU and GPU

So with that being said what do you think is the problem???
Another problem is that when theres a power cut my UPS gives backup but Pc reboots and theres a loop of reboots in 8 mins each until Power is back...
I suspect the UPS bcoz its new and not well known but,
******Can it be a Faulty PSU ? It cant take enough power from my UPS ? Or its not faulty ,but can't give enough power to handle my full system??..with power as well as during power cut???******
OR
Is it a Faulty GPU that i just received?? Changing clocks doesn't change performance? Defective overclocked edition? Can anyone explain why there is no noticeable performance change from stock clocks to lowered clocks? and why is the performance of hd7770 on clocks of hd5450 10 times better and playable? [hd 5450 not playable]
Or is it just that my PSU cant handle higher clocks of my GPU? :/  Which service center should i visit, sapphire or corsair?

Please Reply and Help me _/\_You said:
Quote

1. when i tried my old GPU with overclocked core clock 690 mem 653, i didnt get any reboot during stress testing GPU/ GPU + CPU.