porpoising aircraft

Hello Everybody;
I've been a flight sim pilot -- off and on -- since FS4.0 and I have had an ongoing problem with porpoising aircaft in my sim. By porpoising I mean the continuous up and down pitching of the aircraft as it hunts for equalibrium. A well balanced aircraft can be easily trimmed up for straight and level flight (or any other attitude for that matter). A badly balanced aircraft will continually hunt for equilibrium but never find it.

Although I have occssionally found aircraft that were properly balanced and (properly) flyable, I have sometimes put my simming on the shelf for awhile out of sheer frustration at not being able to control the aircraft in a realistic manner. But, I like flight simming, and after awhile I always come back to try it again.

This porpoising problem has persisted -- for me, at least --over several computer, video card, and Flight Simulator version upgrades and seems to be present in nearly every aircraft I have ever downloaded.

I have used air file editors like AirEd to try to readjust the aircraft's center of balance and/or center of lift, but with little success.

Can anyone help me get rid of the porpoising?
Any and all help will be much, much appreciated!