I just went to your site, clicked lot of links and it didn't skip domain even once. If that's still happening with you try to notice what is triggering it. Its an old board and doesnt really makes lot of sense to behave like that all of sudden.
^ I think his problem is that it reverts to his Frihost url all of a sudden.
I went through your forum to a variety of different types of pages and had no URL jump at all.
Try clearing your cache; there might be a cookie or something doing this for you.
Apparently, the site works fine - as reported by several members.
There's point in trying to solve this, unless the OP can confirm that the problem still exists.
Actually now that I remember (I was there when it was setup)
Orignally one or two attempt's at installing it where from the frihost domain. Quite possibly there was a cookie set, which would automatically move to the proper domain when a new page is loaded.
So as the other Posters have said, clearing your cookies should fix the problem