Performance

My question is on Postgres performance. I have three systems all running
the same database and have the same data feed. One the of the systems is
running 7.4.8 and the other two are at 8.1.4  They all run under Linux.
The type of machines are not really important to my question, since I am
not comparing the performance of each machine. All three machines have
been tuned. Logs do not indicate any problems

The total size of the database is about 500M+ in size. There are several
large tables which are 9M in size. These large tables add and delete
about 200K rows per day. The total database size has not varied by much
over the past two years.

Now my question. The performance of each of these databases is
decreasing. I measure the average insert time to the database. This
metric has decreased by about 300 percent over the last year. I run
vacuum analyze and vacuum analyze full on a daily bases. I also run
vacuum 4 time a day. Still the performance continues to drop. Are there
some other performance activities I may try?. Are there other monitoring
options I can use to indicate where the slow down is occurring?

Thanks Don