High CPU with 7.4.1 after running for about 2 weeks

I've been running Postgresql 7.4.1 for a couple weeks after upgrading
from 7.2. I noticed today that the postmaster had been using 99% of
the dual CPUs (on a PowerEdge 2650) non-stop for the last couple days.
I stopped all the clients, and it didn't abate---even with no
connections---so I restarted the postmaster. Now everything is
running smoothly again.

Is there anything that might accumulate after two weeks that might
cause postgresql to thrash? I'm running pg_autovacuum, so the
database itself should be nice and clean. It isn't connections,
because I restarted the clients a few times without success. I've
been running a long time on 7.2 with essentially the same
configuration (minus pg_autovacuum) without any problems....

Thanks for any help,

-Mike