i have a problem with vista home premium hibernation

hello.
i have a problem with vista home premium hibernation.
i have a hp pavilion notebook and i recently change the graphic card (GF 8400). after that some times when i am working with my notebook, it suddenly hibernate.
may anyone tell me what is the problem?

Hi welcome apologies,
This link may help you:-
How to disable and re-enable hibernation on a computer that is running Windows

Did you make sure when you installed the new card GF8400 that all drivers etc., had been installed?

In addition to the above this tutorial may be of interest.
http://www.vistax64.com/tutorials/63567-power-options-sleep-mode-problems.html
Follow the procedure for troubleshooting sleep problems

and

Go to search, type device manager. In DM make sure that the graphics driver is the most up to date and does not have any yellow waring marks by it.
Update to the most current driver by going to the website of the card manufacturer.