What could cause the database to lock up and queue up all the queries?
You'll want to check the lock details (pg_locks: see "Monitoring Database Activity" in the reference manuals) and also what the system as a whole is doing (vmstat/iostat).
I seem to recall some configuration of ext3 could cause bursts of activity with certain write patterns, but don't recall PG being affected by this.
There have been discussions of context-switching issues with Xeons IIRC - don't know the details, so check the -performance/hackers archives for details.
HTH -- Richard Huxton Archonet Ltd
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