low performance

I am running the precomplied binary of Postgreql 7.1.2 on a Redhat 7.1 (on a Dual Celeron System with 256MB, kernel 2.4.4 and 2.4.5) System.
(The installation of the new 7.1.3 doesn't seem to solve the problem)

I am connecting to the DB with a Perl Program (using Perl 5.6.0 with DBD-Pg-1.01 and DBI-1.19).
The program inserts some million rows into a db with about 30 tables. The processing takes (if everyting works fine) about 10 hours to complete. Usually the my Perl-Script and the database share the available CPU time 50:50.
But sometimes the database is very slow eating up most (>98%) of the available CPU time.
(Of cause I know VACUUM and VACUUM ANALYZE, this is not the problem).

The only thing that seems to help then, is killing the perl script, stopping postgresql, running "ipcclean", and start again from the beginning. If it works from the beginning, the database is ususally very fast until all data are processed.

But if someone else connects (using psql), sometimes the database gets very slow until it is using all the CPU time.

There are no error messages at postgres-startup.
I already increased the number of buffers to 2048 (doesn't help)

I cannot reproduce these problems, sometimes the db is fast, sometimes very slow. The perl script doesn't seem to be the problem, because I wrote all SQL Commands to a file and processed them later ("psql dbname postgres < SQL-File").
Same thing: sometimes slow sometimes fast.

Andreas