Can you please tell me how my pc is able to run on 350W UPS ??
I dont understand these logics if my pc is getting 600W of power when using GFX how my UPS is capable oh holding such load even it is rated to 350W
Your PC may not be able to run on a 350w UPS system when it is under a heavy load.
Your PC will not pull max power all the time. It will pull the power it needs to perform the tasks you tell it to perform.
When you are just sitting there reading an email, your computer probably pulls less than 350w so your UPS should have no problems. When you start doing something that requires the computer to draw more power, it may draw more than 350w...
THEN your UPS could have a problem supplying the needed power to the PC and you could see symptoms just like the ones your are describing to us.
Not all the time. Sometimes.
Power comes from your wall socket, to your UPS and then to your PC.
Your UPS can't pull more power than the wall socket is capable of providing. (Most wall sockets have more than enough power for any consumer PC)
Your PC can't pull more power than your UPS is capable of providing.
It doesn't matter if you have a power supply in your PC that's rated for a million gazillion watts. If it's plugged into a 350w UPS, you will run into problems (like the symptoms you describe) long before you get anywhere near your power supplies rated output or the 600w your GFX might be capable of pulling.
What I'm explaining here isn't the only possible cause for your problem, but it's very high on the list of probable causes.