OK this is a problem that is not real common and is usually a Windows problem the link below will take you to Norbie's World it is a free computer on line diagnostic site I occasionally go to the site and read the forums when there is a problem here I can not figure out.
You will have to register there and they will have you run some test for them and diagnose you problem. And hopefully find a solution. Could you post back here and let us know how it goes.
http://www.norbiesworld.co.uk/computerhelp