Guys this may be affecting some other people's planes or not, I am not sure. Either way. I downloaded a SWA 737 (duh) a while back. When I turn the airplane using the autopilot, at any altitude, it starts violently pitching up and down and the descent rate increases steadily until it dives at around 10 degrees nose down. Even as it reaches the target heading, it continues doing this, and I continually have to recover it by hand. I thought maybe it was running outta trim, so I increased the throw, but it didn't affect it. Any suggestions please?

It doesn't stall. It does it at any airspeed. 130 knots on approach or 270 at crusing...it doesn't matter.

You can also try using fsedit that came with FS2002 just click on the plane in Question than edit it I have found that in the flight tuning section and in the weight section(moments of interia section) that there are some huge differences as to what it suggest you should use.
  But if you can not correct it sounds like to me that it is headed for the recycle bin.

This is true sometimes it is. But in one case I think some one had nervous fingers as they had 300,000 for 30,000 That is the kind of mistake I was refering to. I would not think that the difference would not be that great. But have not really paid that much notice to it. It was just one airplane and changing that entry to what FS$ suggested sure made it fly like a plane rather than a boulder.