System Restarts when connected in UPS during Gaming

Hi all:

Please suggest me an appropriate solution.
My PC restarts when it is connected to the UPS but runs good when directly connected to the main switch.
The restarting problem occurs only when I play Intense graphic games (Farcry 3 ASS 3), during normal operation of using Excel or viewing film there's no problem of restarts.. I've done no overclocking

My Specs are :

ASUS P5N73 AM MOBO
Intel Core 2 Duo
ATI HD 5770
Huntkey 700W SMPS

Just simply don tell me tat UPS is the problem, Because I was also assuming the same and bought a new
1 Kva SMPS Numeric last week but it continues to restart.

Just because it is new I dont say tat its good I've also checked with an electrician and he says that UPS delivers an output of 230V, So there's no problem in it.

But wats my question is if my SMPS conversion efficiency (AC - DC) is too low so that it pulls more WATTs (May be more than 600W to provide the required 400 W/ 450W (Full load)) than Rated of (600W for a 1 Kva UPS).

Or If u think UPS is the problem I've checked wats the output Volt from the UPS? If there some other ways to check out what continuous watt the UPS delivers in Full load??? (It must deliver more than 600W)

Consider in mind that my system requirement during full load is only 400/450 W (Sources from PSU calculator in Extreme Calculator, ASUS website etc)

Suggest me a good solution.. But also Please Justify ur answer so that I could proceed further..

For Additional Info I've ran Memtest86+ (2.5 Hrs) and Prime test 95 (3 Hrs) when the system is connected in UPS and it showed up no errors also there's no proble of restart. To monitor heat I've HW monitor and Speedfan which shows a proper temp of below 53C for Gr Card & CPU 40 - 44 C) which is fine so there is no cooling issue i guess..