Steve:
you need to contact simware to see where you can get the updated version. or did you just download the FSX version from simware? i'm not sure as you wrote -
you wrote -
reports of this behavior have usually been related to a computer lacking enough CPU, rarely the GPU as FS is CPU dependent, to process the game (ie, screen refresh problems).
did you purchase a new computer with modern powerful CPU? a computer able to run FS9 may not be able to run FSX.
another issue is background processes slowing the computer (ie, running a www-browser while playing).
also -
steam version can act differently than FSX so just installing a FSX version into steam can cause unexpected problems.