query is taking longer time after a while

I have a table with 5 million rows. 10 inserts and deletes are performed on
this table every second.
The table has indexes on the columns I use to query it, and the query is
returning about a 1000 rows. Initially the query takes a very short time
(order of miliseconds), after a few hours it takes hundreds of miliseconds,
and after a few days it can take more than 10 seconds. When this happens it
also blocks all other operations on the database and I see very long times
for all of them.

I thought this may be caused by the indexes not remaining in the memory, but
I increased the shared_buffers to 0.5 GB and this didn't seem to help.
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