Issue with GALAXY GTS 450

Hello everyone,

I have assembled a pc recently by myself. The configuration is as follows: -

Intel i5 2400
motherboard - intel DH61WW
500 gb sata seagate hard disk
Galaxy GTS 450 GC graphic card
basic PSU unit - 230v 5amp, +12v rail has 22 amps
ups - frontech 600VA

This pc has been assembled just three weeks back, i have windows 7 ultimate and i had no issue with the pc initially other than heating problem while playing games (games used to shut down when the chipset use to get heated up), i resolved this issue by using speedfan to increase the speed fan manually and leave the side cover of the desktop open while playing. But recently i had some problems with the desktop, the pc used to restart after logging in, before showing the desktop screen. The moniter used to go in power saving mode and nothing would respond and used to restart by itself. This has to happen every first two times when i use to start the pc, one more problem i use to face during this time is when playing games the pc used to restart with the moniter going to power saving mode. I tried updating windows and the restarting problem got resolved, but still while playing games the pc used to get hang-up. I tried uninstalling graphic cards driver reinstalling but had no positive results.

PS: I had little confusion when installing graphic card on the motherboard, my psu comes with 2 peripherals connectors and out of which one goes to the CD-Rom and i have one remaining,and PSU has none 6-pin PCI connector. Now my Galaxy graphic card has 6-pin PCI connector but a supplementary y-peripheral connector has been provided with 3 pins each, which can be connected to the 6-pin pci connector. Before installing i had checked out in different forum about this extension and someone had commented that there is no need to connect both the peripherals but just one will do. Therefore as i had only one peripheral left with me, i connected it to one of the y-peripheral and left the other. Everything from Grid to Battlefield Bad company 2 was working fine untill recently when it started making problem by getting hanged-up. The basic graphic while playing movies or the desktop visuals dont have any issue other than gaming.

Any suggestions how to solve the issue?