Need more info.. first is this a newly installed game? if so, check the hardware/software requirements on the game box, does your pc meet these requirements. One possible issue that pops in my mind might be memory issue or possibly your pc is under power. Video card issue normaly would not restart your pc not unless you installed a new video card that cosuming power that you power supply can't provide.
Please provide the above question first.
Yup! It's a newly installed game but there are no problems in meeting the system requirements of the game. My power supply provides 550W i think.
Are you having a problem with just this new game or any online game that you played? Can you provide me the name of the game you're having an issue with? Did you just replaced your video card? I ask about your video card cuz you mentioned that you suspect it might be your video card. What is the spec your video card? How much memory do you have?
Which operating system do you have? Check your event log to find the error code (if one exists). Post the error code here and I'll take a look. It never hurts to upgrade all your drivers, specifically the video driver, Nvidia and ATI both regularly release new drivers that address specific game issues while typically providing greater overall performance. I know the Gforce8800 has had some problems with a previous driver.
Some hardware, however good, just doesn't work with some games. The developers develop for certain hardware and cant take everything into consideration. Sure they are moving over to multiplatform development, but there are still a couple gaps.
The only advice I can give you is to contact the support team for the game, give them your specs and see if they have had a similar problem somewhere. Maybe they can help you.
If it is not that then something might be clashing with the game. Directories, permissions, firewalls, antivirus, hardware, graphics drivers, direct x, etc. Maybe your graphics card is loose or needs replacing.
There is a lot that could be going wrong. I hope this helps a bit. Good luck