OK. We might have explained the problem a little bit wrong.
When we go to for example edit hooks in the admin cp, then the server/database starts hanging.
At first we couldn't even go there. We had to adjust the php memory limit from the default 128MB to 256MB. And now we can go there, but it's very very slow. And also the forum stops responding when we do it.
So in that way we think it's a database problem. But it could be connected to the server and not the forum software.
We think there may have been some problems in our data base during one of our forum upgrades so we want to install clean forum. Is it possible to move forum members and imports/ export posts to a new clean forum?