GTX 480 + Q6600 + FS9.1 problems

Hi there, SInce installing my GTX 480 on my computer I've always seemed to have one or two nagging problems. The one I've experience lately is the most frustrating, and I've run out of ideas as to how to isolate and deal with the problem. Problem:When changing view on the view hat (or even during taxi, when turning) my FPS will suddenly begin to drop rapidly, and become stuttery (from 60 to ~ 15 FPS). It clears when I hit the ALT key and bring up the menus, and FPS return to 60. It'll continue to happen every so often, until I hit ALT again. My set-up Q6600 (overclocked to 3.8MHz)EVGA GTX 480Windows XP, Service Pack 3nVidia driver Version: 6.14.11.9975 (newer drivers introduce all kind of strange effects and artifacts - it was my understanding that using older drivers for FS9 was the only way to go, and that has been the most stable driver I've found)FSUIPC registered versionFSPassengersFSCaptainDBS Walk and FollowHundreds of add-on airports and freeware AI traffic packages. I've disabled all my add-on scenery, and the problem still occurs. I've disabled all AI traffic and the problem still occurs. I've tried removing the .dll for FSPassengers, FSCaptain and Walk and Follow, and the problem still occurs. I would very much like to avoid doing a reinstall, as I've hundreds of add-ons that I've purchased at huge cost and many hours of installing and tweaking. If this is my final recourse, I would seek advice at how best to do it with an eye to getting everything up and running again. Anyone have any ideas at what I should do next? Thanks in advance for your help. - daniel