stats collector suddenly causing lots of IO

I have a lot of centos servers which are running postgres. Postgres isn't used
that heavily on any of them, but lately, the stats collector process keeps
causing tons of IO load. It seems to happen only on servers with centos 5.
The versions of postgres that are running are:

8.1.18
8.2.6
8.3.1
8.3.5
8.3.6
8.3.7
8.3.8
8.3.9
8.4.2
8.4.3

I've tried turning off everything under RUNTIME STATISTICS in postgresql.conf
except track_counts (since auto vacuum says it needs it), but it seems to have
little affect on the IO caused by the stats collector.

Has anyone else noticed this? Have there been recent kernel changes
that could cause this that anyone knows about? Since we haven't touched
postgres on these boxes since they were setup initially, I'm a bit baffled as
to what might be causing the problem, and why I can't make it go away short of
kill -STOP.

Any suggestions would be much appreciated!