Heavy load, that's all... It starts to happen after a hundred thousand members or soRoll it up's whole webiste has been down repeatedly the past few days
Have you considered pruning your database? I'm sure there are a ton of inactive users that haven't logged in for a crazy period of time.
The board I administer I prune the DB ever 6 months or so to keep the database as lean as possible and maintain uptime.
-RT76