FS2002 on new computer -- sound distorts "after a while"

I have successfully used FS2002 for the past 2 years with a P3/1000 and a Creative Vibra128 soundcard. Recently I built an AMD XP2400 with everything upgraded including video card -- except I purchased another identical soundcard. Well, I say identical at least in name -- the card looked a lot diferent to me.This computer is much better for FS2002 -- much smooother, much better FPS , etc. EXCEPT -- every flight after some time - the sound suddenly distorts and I get the famous " FPS changes with sound on/off problem". Now what I can't understand is this -- if the soundcard is an elcheapo one and no good - of if some setting like sound acceleration is incorrect-- why doesn't the problem occur immediately on startup -- why does it take some time to appear??Any ideas?Thanks Barry