statement stuck when the connection grew up to 45 or more

Hi,

I'm using Postgresql 8.1.3.

Recently I facing one problem, when the connection for postgresql grow up
to 45 or more, when I trigger a statement from WebApp
this statement will stuck forever.
I try to kill this transaction and then trigger the same statement again
but it still the same.
But this time I leave the transaction there and try to kill other
connections that is not in use.
It's weird that after I kill around 5-10 unused connections, the statement
start to run and finish.

Could anyone give me some idea how could this be?
Is it a bug of PostgreSQL?

This is the statement which having problem:
      select count(distinct empno) as counter1 from pay_master_history
            where empno in (select empno from pay_batch_basic_history where
organizationid like '015003%')
      and processyear = '2006'
      and processmonth = '05'
      and processbatch = '1'

Thanks!