Well, the server used to have 384 MB memory and was a Pentium 2 233MHZ, not a speed demon really. =] We have about 3500 users and a couple of hundred posts a day. It's stopped rising for some time because the server couldn't take the load. Probably was mysql tuning issues as well. The weird caching issues could also have been a strange perl/mod_perl install as well, not really sure.
The phantom posts sounds like the problem we had when the mysql server was hogged down. I think the problem is that one of the last queries of addpost.php sets the B_Main and when there aren't enough open connections it can't run that last query. Or, the query dies while executing or something like that. It always happened when the server had huge load anyways, don't know exactly what caused it though.
Don't know why B_Main is set on its own at the end of the script either, but I hope there is a good reason for it even though I couldn't find it when I looked if I remember correctly.
Now, with an Athlon Thunderbird 1200 with 896 MB memory it seems to run very smoothly with ~40 users online. And we have a bunce of other sites running as well, where one is as big or bigger than ours.
