stuck spinlock in postgresql-6.5.1

I have updated my DB from postgresql-6.4.2 to version 6.5.1 to slove
the stuck spinlock problems. But it seems not work.

I have both large of data to be updated to DB "all the time" but also
have large amount of proccessess to access (update) the database
simultaneously.

If the number of processes is below 6, everything work fine. However,
when the number of processes accessing the database increases, then,
stuck spinlock happens soon after all the processes start.

Because the accessed rows by these processes is mutual exclusive,
I do not have any "LOCK table" or "SELECT FOR UPDATE" command in
all the process.

I have updated the postgresql to 6.5.1 but stuck spinlock happens.
Does anybody know that whether the problem is caused by database,
or by the database application written by me?
Thanks
Y.J. Chu