Erroneous Headings Indicated

I am quite honestly at the end of my wits with this issue. I'll try to explain as best I can. First, my system: FS9.1Windows XP Media Center Edition SP3Gateway 825GM (2005 era)Intel PIV 5503.4 GHz800 MHz FSB1 MB L2 CacheATI Radeon 700 series (720 I think)Active Sky EvolutionVarious Addon Scenery, both Payware and Freeware (most from AVSIM) I usually start FS at U76 in Idaho (Mountain Home Muni) on the Active Runway, which is usually Rwy 28 because of the prevailing winds. Heading for Runway 28 is 282 degrees. I used to start with the default B734 from FS9 and then switch to whatever aircraft I choose to fly, moving to the airport of choice with the "Create a Flight" option. Randomly, my heading indicator will sometimes read 282 degrees and sometimes 296 degrees. To correct the bad reading of 296 degrees I have to end my flight, go into my scenery library and disable any addon scenery I choose, then exit FS9 and restart. The scenery I disable does not matter. It can be one I have installed that is halfway around the world 2/3 of the way down my addon list or it can be the one first on the list. Doesn't matter. All I have to do is disable one and restart...voila, 282 degrees on the heading. It used to only occur with the PMDG aircraft I have installed, but now it does it for everything, including default aircraft. I have downloaded and installed the MagDec2005 fix for the magnetic Deviation correction in FS9. I have disabled and re-enabled all 397 of my add-on sceneries. I have aligned the IRS in the PMDGs At one point when I thought it was just PMDG specific I posted in the PMDG forum here: http://forum.avsim.n...path-deviation/ Now it randomly affects all aircraft it seems, even the default cessna I load up with. A fellow pilot on VATSIM said it was ASE causing the problem and there was a fix involving FSUIPC in the PMDG forums. My problem is not "S-turns" capturing the flightpath, it is a consant deviation from the flightpath to the left of the path between waypoints. Plus my aircraft heading is 14 degrees off the predicted flightpath. The aircraft flies in an "arc" between waypoints when using an FMC or the default flightplanner in FS9. I cross all waypoints, but as soon as I cross one the aircraft immediately begins the left side drift, sometimes up to 7 miles off path to the left. With the PMDG this "waypoint" can be an actual waypoint or the TOC indicator, a speed reduction indicator, the TOD point, or the E/D point. Another pilot said I hadn't updated my magdec, but I had done that already. Yet another pilot said it was a corrupt scenery file, thus my disabling all my add-ons, and then adding them back in one scenery at a time and checking to see which scenery file was corrupt. I ultimately got every one reinstalled after 4 days of enabling, ending FS9, restarting, checking the heading, enabling the next one in line, ending, restarting, etc, etc, for 397 files!!!!! After I was done I had every thing I disabled, enabled and within 2 flights my heading was back to 296 when it should have read 282. Still another said it was a bad AFCAD in my Addon Scenery/Scenery File. I've checked them all and tried every fix that has been suggested. I even ran the FS9 Registry Repair Tool. I am lost!!!! I honestly don'y know where to turn except here. Please, my system, as archaic as you may think it is, ran things fine with 50-60 FPS with ASE, a payware aircraft, in traffic, on VATSIM, during events, most recently the KZMP FNO on VATSIM. For that one I only had MSP, my departure airport, and BOI, my arrival airport enabled and the PMDG B738 tracked flawlessly. So please don't suggest upgrading to FSX, a dual core, super blue, 18 gazillion gig system. Not an option. Cash is tight in this economy, okay. It worked before (over 1,000 hours on VATSIM) so what happened? I've been doing this for almost three years now and this problem is the only one I haven't been able to find a fix for. Help, please!!!! Randy