I know the reason choppers do it, the veering thingy, is due to the main rotor torque not being balanced properly in the models by the tail rotor. This is actually a correct behaviour of helis, and you can trim the yaw out once you acheive cruise with your rudder trim. The SH-60 and other modern helicopters don't have this problem to the extent others do due to the canted tail rotor. There are ways to change the .air and Aircraft.cfg files to make it fly straight at cruise speed, which is normally about 10kts IAS below Vne. There are articles on Hovercontrol's website that explain this in detailed, yet very simple terms, so morons like me can understand it
The HC website also has a nifty program, HelicopterTotalRealism, or htr1.5 that you can run alongside FSX or FS9 (also called FS2004 COF), that adjust many of the helicopter parameters in realtime as flight confitions, power, collective, cyclic etc etc change, using FSUIPC.dll. Another program that does the same thing, but IMO not as well, is Heliforce 1.0, also available on HC.
HTR also includes a wonderful PDF file the explains how it does what it does and what effect changing certain paramerts will have, basically a tutorial on "How Heilcopters Fly".
As to the fixed wing, could it be the P factor is modeled a tad too high on that model? Or perhaps it's accurate to the plane and OTHER fixed wing birds don't model theirs high enough. Or perhaps you have changed the P factor slider lately and didn't realize it? Those are some alternative explanations I can think of right off, I hope something in all this babbling helps you
EDIT:
OOPS! I just reread your post....is the plane a jet? If so, then P factor isn't a factor at all! LOL! Sounds like a W&B problem. Did you look at the Aircraft.cfg file to see is there is an inbalance? Perhaps one wing has a load the other doesn't, the COG is in the wrong place? Typo's happen and it could be the author messed up someplace? Maybe a problem with teh .air file's stabilizer settings too? Just suggesting possibilities....
I'll look at the plane in more depth tomorrow, now I am off to bed with my lovely wife
Good luck and happier flying!
Pat~
,, Have you guys heard of such a thing or had it happen to 
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