I'm not too sure on this. You said that your internet disconnects after about 20 mins - it could be that your modem is not disconnecting properly and that your computer is using up all of its resources to try and disconnect it. I would try reinstalling your modem driver but I don't think it would make a lot of difference.
PS: do you have any other versions of flight simulator? I have got 95, 98, 2002 and 2004