PC volume disappears automatically any time of instance

Good evening friends!
My PC volume disappears automatically.
After few minutes I start my computer my PC volume disappears & clicking the volume icon shows you don't have any sound card installed.
But I have my sound card installed.
I check for the recent driver for my motherboard & installed it but still problem persist.
But after I restart the computer sound comes for a few minutes & then disappears.
I am using windows 7 dual core processor & D945 gcnl motherboard.
Help me out please.
