Have you tried to run an offline mission in the channel map?
here you are a link for downloading:
http://airwarfare.com/sow/index.php?...tegory&catid=7
Try with the large one first, and if its ok the huge.
If you can run them, I dont think you have a memory issue.
last resource: reinstall the game. I've seen other people having success with this option
Wish you luck Cazador, I know is a pain the process, but its also hard to me to believe is a memory problem.