Ah crap, Urchin you have the same problem as me. To be honest I never pinpointed a source, but I following may variables may be the culprit:
1) orientation of the tracking camera - mine used to slide around due to lack of USB cord length... until I taped it down.
2) inconsistent location of head - my chair is on wheels and can't seem to 'find the sweet spot' in relation to the camera every time.
3) position of hat and clip - the clip would fly off at times and it would not be centered on the brim of my hat. Also the centering of my hat would also change, so the clip position would again become incorrect and off center.
So basically the setup would have very minor alterations each time I got settled in to fly. These alterations would cause huge annoyances. I discovered the comfortable position of a hat on my head never really had the center of the brim in line between my eyes and on the center of forehead, so that was a starting point. It drove me nuts for a while until I realized there was also a big difference in the camera position as well as my seat position in addition to the hat/clip orientation.
I had to get used to the software update a few years ago (2004?), and after that it never felt like my yaw and pitch were moving with respect to pivot point, rather it felt as if this point turned into a point along a radius in front of the point, causing annoyance. And more recently it became asymmetrical and inconsistent -- I would look right and it felt very good, very natural -- then I would look left and it would do the 'back up into cockpit glass' deal. If any of the aforementioned variables were not right, I discovered, the 'pivot around a radius' would be exaggerated along one half of an axis.
I never did find a method to finding my sweet spot. Every so often I'd sit down in a particular spot, adjust my hat, adjust my clip, and if need be, adjust the camera position on top of my massive CRT (I now have an LCD and it no longer moves), and everything would work out. I can only suggest messing around with every variable until you find something that works good for you. Once you find it - take a sharpie or tape and mark every position -- your hat, clip position on hat, camera position, chair position, position relevant to moon, position relevant to sun, position relevant to pluto, etc. Then simply duplicate the process each time you fly.
Sorry for the wall-o-text, but it really seems like it's hit or miss with this stuff.
[edit: Sit down for big adjustment sessions with AH closed. Use the 'view head' to compare all six axes, you'll be amazed at how off it is between physical adjustments in front of the camera.]
