pgsql upgrade

Hi all, I've run into an issue with a Pg 7.4.6 to 8.1.5 upgrade along
with hardware upgrade.
I moved the database from a 2x 3.0ghz Xeon (512kb w/HT) to a 2x Opteron
250. The database is in
memory on a tmpfs partition. (the application can rebuild the db during
total system failure)

When I first switched it over, the results were exactly what I expected.
I was sustaining about
a 2.2 on the Xeon and with the Opteron, about a 1.5 with the same
traffic. The box is highload,
it ouputs about 15mbps.

After a couple hours working perfectly the ' system' (vs user) load
jumped from
a 3% of total CPU usage, to 30%, a 10x increase, and postgres started to
write out
to data/base at a fairly fast rate. The CPU context switch rate doubled
at the same time.
Iowait, which was historically 0 on the 7.4 box, went to 0.08.

Strangely enough, a vacuum (not full or analyze) stopped postgres from
writing to data/base
but the strange load pattern remains. (system is ~30% of the overall
load, vs 3% before)

So, my question is, what happened, and how can I get it back to the same
load pattern
7.4.6 had, and the same pattern I had for 4 hours before it went crazy?

Matt