Turns out we did experience some slowness yesterday. It wasn't our little server's fault actually, this time it was the entire server neighborhood we were in, so out of our hands, so to speak.
Unrelated to this, we're planning another server move real soon, which should speed up the forums a great deal (fingers crossed!).
I've seen it do this too. Actually there are times I time out completely trying to get here. But i try again a few minutes later and it's OK.
Purely a guess, but as someone who runs a hosting company on the side for cluster services my personal server (big beast) had a tendency to do this as well. when running cron jobs primarily for backups, log processing, etc. My 8 processor server is dedicated to a single site that gets millions of uniques per day. So tracking server load becomes a full time job.
My guess is it's a services issue because it's not steady, it's at odd times and quickly clears up.