Right now the forum performance is fairly horrible again. I'm clicking a link and have to wait about 30 seconds before anything even starts to happen, sometimes it takes even longer than.
And I'm just reading not doing anything crazy.
I agree with PeterSmith, this feels like a database issue somehow. Is the forum software maybe not relinquishing server connections to the SQL server properly when it's done with a session or something like that? (usually shouldn't happen because the SQL Server has a timeout setting too, but maybe that's not working right)
Databases are usually really good at handling huge amounts of data, so it's likely not "there's just too much in there", also then it wouldn't get better again.
Whatever it is, there's definitely a bottleneck somewhere, and something hogging all the database access would certainly qualify.
The fact that it seems to pop up in (more or less) regular intervals suggests it's some kind of "memory leak" issue, where some resources aren't properly freed up again, even though they're not in use anymore.