database troubles - various errors

We're having a lot of trouble with one of our new servers.  It's an AMD64
dual cpu system, running on Gentoo Linux, with kernel 2.6.7.  Data is stored
on a JFS partition.  Before we installed the server, memory was fully tested
and no problems were found.  Postgres is version 7.4.3.

    We've had a number of different errors.  One of the most popular seems
to be a Cannot open segment.  This usually occurs on an index, generally the
primary key index.  A reindex will fix the problem.

    Last night we got this one: ERROR:  could not find a feasible split
point for "some_index".  This one seemed funny because it was on a btree
index of one column.  A reindex made the error go away.  Every table in the
database was reindexed at some point last week.  The table with the index
involved uses all standard data types and has no non-btree indexes.

Another one we've had and fixed was ERROR: duplicate key violates unique
constraint "pg_class_oid_index".

One of the most annoying errors is occassional index corruption that is not
reported.  We make use of gist tsearch2 indexes, and sometimes they will
decide to stop working without an error.  The only clue we have that there
is a problem is that search words that generally return big results will
return any results or very few.  A reindex will fix the problem.

Vacuums are done manually, but generally there's at least one every week.

Basically, I'd like to know if anyone has seen any problems similar to this,
and if they have any suggestions on how to fix these issues.

-Adam Palmblad