MySQL Performance

[Troy Settle]

I've a 4-STABLE box on a Dual Xeon w/4GB and U320 RAID:

  4.11-STABLE FreeBSD 4.11-STABLE #7: Fri Mar 11 20:24:25 EST 2005
  10:05AM  up 55 days, 14:38, 3 users, load averages: 1.07, 1.10, 1.13

Running on this box, is MySQL 4.0.23a (a little outdated, I know). This 
MySQL server seems to run fine as long as I watch it, but as soon as I 
turn my head for a moment, it jumps up to ~97% CPU usage:

  508 mysql     63   0    99M 51660K CPU0   0 306.9H 97.46% 97.46% mysqld

It doesn't appear to be an IO issue, 300 samples from iostat show a max 
of 0.31 MB/s.

It also doesn't appear to be a memory issue:

  867M Active, 2354M Inact, 320M Wired, 192M Cache, 199M Buf, 40M Free
  Swap: 4096M Total, 16K Used, 4096M Free

Through a number of samples from sockstat(1), I see ~20 connections to 
mysql at any given time.  5 for Courier's authdaemon, 3 for Exim, and 
~12 for Apache/PHP.  There are intermitant connections from Windows 
clients using MyODBC, but none are persistant.  Restarting any/all of 
these processes does not cause mysql to ease up on the CPU.  Only 
restarting MySQL will buy releif, but after a few weeks, it's right back 
to 97% CPU usage.

The machine itself is completely responsive, mysql is completely 
responsive.  The tables in question are flat, with only a few thousand 
entries in the largest.  Queries range from simple to semi-complex.

I can't identify the source of the load.  Can anyone help?  What should 
I be looking at?

Thanks,

--
   Troy Settle
   Pulaski Networks
   http://www.psknet.com
   866.477.5638