x-fi - problems with the volume

Hi,

I have the X-fi Fatality sound card and its latest drivers installed.
When it plays a sound on my XP, whether it be mp3, a video or a PC game, the volume goes up and down all the time. It's never stable.
When I try to increase the volume on my Creative 5.1 speakers, the volume goes up way too high and within 2 seconds the audio fades away gradually as the volume somehow goes down and down on its own until you don't hear no sound anymore.

I can't listen to audio on my PC like that, playing with my speaker's volume wheel all the time to hear something.
What could it be?

I tried disabling EAX effects, CMSS, switching to other modes, but nothing works.

My onboard Nforce 4 sound card is disabled in the bios so it couldn't cause any problems.

Could it be an audio codec or something? But then why does it happen in games as well?

Any suggestions would be highly appreciated.
Thanks in advance