Postgres 9.1: Adding rows to table causing too much latency in other queries

I recently upgraded my postgres server from 9.0 to 9.1.2 and I am
finding a peculiar problem.I have a program that periodically adds rows
to this table using INSERT. Typically the number of rows is just 1-2
thousand when the table already has 500K rows. Whenever the program is
adding rows, the performance of the search query on the same table is
very bad. The query uses the gin index and the tsearch ranking function
ts_rank_cd.

This never happened earlier with postgres 9.0 Is there a known issue
with Postgres 9.1? Or how to report this problem?

-Sushant.