Sir Starkers:
I've read all of the responses of this forum and most of these suggestions are good ones.
I have 2 questions though which may have been an over-sight but not mentioned.
First, is this a problem that you've always had, or something that just came up?
The other question is, how are you connected? Analog (6 channel direct) or digital (Coax or Tos-link)?
You see where I'm going with this?
If you've always had this issue, chances are you got a bad sound card. If this is something that just came up, ask yourself what's changed and when.
Also address your connection, being that 6 channel direct is a lower quality and subject to all sorts of outside interference whether it's directly connected to your system or not.
So if your running analog, you may consider a digital connection.
Just trying to give you more ammo!
G'Luk my friend.
Cp