suspicius behaviour during delete

Hi all,

I hope this is the right mailinglist for my question.
I'm using postgresql 7.2.1 and doing some bulk-loads from one table
to another. Due to sometimes there may exist som already loaded rows the
first thing I do is to delete them to reinsert all of them later on.
This sequence is but into one transaction.

My problem now is, that postgresql behaves somehow strange. Sometimes
when I practice like mentioned above it works just fast within seconds
but sometimes it last more than an hour. Both times 30000 rows are
processed.
Even if I'd like to delete and insert only one row it lasts an hour
until I cancel the statement.

Does anyone have an idea or can anyone give me some hints how to solve
that problem?

I'd appreciate any help!

Thanks in advance,
Yann