I think you will find that a lot of providers impose a 2 hour cut-off Shaz. :'( One which will let you stay on forever is AOL but to be honest, I don't think it's worth putting up with its idiosynchrosies and restrictions. For example you can't use Outlook Express with it.
If you are happy with Freeserve and can log on easily each time you want to, why don't you log out and log back in again before you download a big file? That's what I would do anyway.