this usually happens when a server encounters a high load and your SQL queries don't make it into the stack because the SQL servers stack is full.
You can do 1 of a few things to fix this generally.
1. Get a more powerful server.
2. Resetup MySQL/SQL Server to run in a more 'industrious' mode. (Unless you're already at the top then you may want to start running clusters)
3. Optimize the queries in the software (which phpbb already has for the core of it)
Whats most likely happening is occasioanly this site gets a spike and a few users loose their queries resulting in that error, by the time you have refreshed the page the load has dropped and your queries are now in the stack and are processed.
Anyways - a person doesn't always fix it - the load of the machine, the application restarting itself, the application forking a new process can all play a part in it.
Generally your webhost will monitor services like SQL and are notified if it is down for 15 seconds - 1 minute etc. (generally depends on the quality of the webhost and the level of your service)
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