Sound and Video Issue

Hey folks just wondering if anybody has any ideas on a sound video issue I'm having, this could be as easily posted in the software forum so forgive me, I just don't know what kind of issue it is. I run an 8800gt, e8400 cpu, Asus P5KE-Wifi and 2 gig Kingston RAM. I also run flight sim , and this is pretty much the only reason I have a PC. For the last few months now when 3D graphic rendering is occurring the sound just crackles like mad, and the frames in the flight sim go to single figures. This would usually be solved by turning PC off for a while and turning back on. Now the problem is constant. Even when playing a CD or mp3 in windows media player, it plays fine if I'm looking at the library but when I switch to a visualization it almost stops. Also when I scroll up and down in internet explorer it'll cause sounds to stutter. In the sim when I'm looking at heavy load scenery it'll be on its knees but if I look at something not intensive eg the ground it'll be fine.

I yesterday installed a sound card and disabled on board sound believing this would solve my problem but it seems to have gotten worse. Last night I discovered that if I use my usb headset, music playback is fine, even with heavy visualations, and in the sim the sound is fine but the frames are still like a sideshow. Apologies for the long winded explanation but if anyone has any ideas even obvious stuff that I may have missed I'd be very grateful.

All drivers are installed correctly and up to date.
Regards
George