I've been having the exact same problem, but I think I found a solution. I have installed Jungman's sonar pak, which apparently replaces a screwed up version of the AI_Sensors file, when you are using RUb. If you are not using RUb, then this may not be the problem, but you might want to try it anyway. You can check the history of this topic here:
http://www.subsim.com/phpBB/viewtopi...708&highlight=
At the bottom of this thread you will see a link to the Sensor Pak thread. On page 5 of that thread about 1/4 of the way down, there is a link to his Sensor Pak.
Hope this helps.