Upgrade the sound driver. That's the first place to start. You can also try to upgrade the Motherboard firmware if that doesn't work. You may also want to disable the onboard sound through the bios if you are going to use a PCI card. Sometimes there may be a conflict with the drivers. You can do this in the BIOS. When the computer is just starting to boot, you'll want to either press the delete key or the F10 key (read your motherboard guide for this info).
At one time I had a conflict with a SoundBlaster card and and IDE PCI controller card and had to switch to the MB sound system. I switched back and the conflict was gone.