[CentOS] IBM x305 slowdown?

Hello:

I have an odd problem with an IBM EServer XSeries 305 (aka x305). 
Sometimes it seems to just sit there, completely unresponsive to the
command line (ssh and console), for several seconds.  Other times it
works normally.  top will sometimes die with a floating point error
while waiting for it to start.  When top does work, it often shows a
system load of 0.5, with high interrupt load.

At first I though that the tg3 driver was not playing nicely, so I
disabled the onboard NICs and installed a 3Com card.  This seemed to
clear up the problem, but it's now returned two days later.

System is running CentOS 4.4, no X, Apache and MySQL.

CPU is a P4 1.8GHz
512MB RAM
40GB ATA-100 disk

Any suggestions on where to look next are appreciated.

--Chris