sound slowing down

Whenever I am playing a game, the game and the sound slow down to the point where the computer almost freezes. Then all of a sudden it runs fine. This sometimes happens when i watch a video as well. It happens on every game from command and conquer red alert 2 to age of empires 3. It happens continuously when I play.
I have a compaq presario 5000 with
1.5 Ghz Pentium 4
512 MB RAM
256 MB DDR ATI Radeon 9550
My speakers connect right to my motherboard