Sound/Graphics problem

I have developed a problem over the past couple of weeks. After FSX has been running for some time - 30 minutes or so - sound develops a repeated crackling and frame rate drops suddenly for a minute fraction of time. If I am running a mission with sound commentary, the speech develops a stutter and repetition which can make it unintelligible. I've tried dropping the sliders back, but this has no appreciable effect. Is this a sign of something horribly wrong with my setup? I don't get this problem when playing a DVD for example.Any suggestions will be most gratefully received, 'cos it's driving me nuts!!!jaje