Disk filling, CPU filling, renegade inserts and deletes?

Hi,

I'm having a pretty serious problem with postgresql's performance.
Currently, I have a cron task that is set to restart and vacuumdb -faz
every six hours. If that doesn't happen, the disk goes from 10% full
to 95% full within 2 days (and it's a 90GB disk...with the database
being a 2MB download after dump), and the CPU goes from running at
around a 2% load to a 99+% load right away (the stats look like a
square wave).

So it's problem-hunting time, I guess. The problem has something to do
with the following errors (there are a lot; I'm posting a short sample)
NOTICE: relation "pg_depend" TID 43/27: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_depend" TID 43/28: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_depend" TID 43/29: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_depend" TID 43/30: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_depend" TID 43/31: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_depend" TID 43/32: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_type" TID 17/44: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_type" TID 17/45: InsertTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/11:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/12:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/13:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/14:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/15:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/16:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_attribute" TID 133/17:
InsertTransactionInProgress 209545 --- can't shrink relation
NOTICE: relation "pg_class" TID 41/18: DeleteTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_class" TID 41/19: DeleteTransactionInProgress
209545 --- can't shrink relation
NOTICE: relation "pg_class" TID 41/20: DeleteTransactionInProgress
209545 --- can't shrink relation

When I vacuum full, I can't get rid of these errors unless I restart
the database (and then I restart, vacuum full, and everything's fine).
And once I do a successful vacuum full, CPU usage returns to normal,
and the disk is no longer almost full (back to 10% full). I'm at a loss
to figure out where the problem is coming from and how to fix it.

My machine: XServe G5 Dual 2GHz running Mac OS X Server 10.3.9.
Postgresql 8.0.1

Thanks for any responses/ideas/solutions (best of all!),
Richard