anomalous, persistent high loads with postgresql 6.5.1

I've searched the list archives and read the FAQ, neither of which seem to
describe my current situation:

I'm running Linux 2.2.10 on a production webserver, along with PostgreSQL
6.5.1 (in RPM format from ftp.postgresql.org). My main database is a set
of names/addresses/etc. tables for our membership, about 60,000 records
apiece, with indexes in all the right places. Whenever I run any sort of
involved query, the postmaster process kicks into CPU-eating mode; even
after I've made sure no more queries running (including the original),
postmaster/postgres processes keep the system at a load average of 3 and
rising; the only way it seems to subside is if I kill and restart
postmaster, which is obviously a problem ^_^;;

Is this a weird OS/dbms/query combination bug, or am I just doing
something horribly wrong?