my computer configuration,
AMD phenom[tm]llx6 1090T processor 3.20Ghz
mother board M4A89gtd pro/usb3
OS Windows 7 ultimate 64bit
graphics card GTX550Ti 1GB/DDR5
i use adobe premier cs 5.
graphic card does not help to render fast.i unlock the card.when i render cup is 100%
when i overclock computer gets shutdown
Could be the size of render going beyond your 8GB's of RAM. With more RAM you can render much larger compositions. With more RAM you would have more space for caching items. The project’s color bit depth my be too high or require more RAM. Your 6 core process could be getting in its own way. The more process you run at once can limit the amount of RAM to each slowing things down. You could limit it to say 4 cores to test this.
I suggest getting more RAM if your render quality is important.
I was using Premiere CS5 (recently upgraded to CS6) and can testify using CUDA from Nvidia helps a lot with rendering. As was previously mentioned more RAM does help and you should see a difference going from 8GB to 16GB. Have you check the setting to see if it is using Mercury Playback Engine GPU acceleration? (Project, Project Settings, General).