How do I track down a possible locking problem?

I have a production system using Postgresql 9.1.2.

The system basically receives messages, puts them in a queue, and then several parallel modules, each in its own
thread,read from that queue, and perform two inserts, then release the message to the next queue for
non-database-relatedprocessing. 

Today, after we received complaints from a customer about delays, I noticed odd behavior in the system. About 2 percent
ofthe messages were inserted into the tables more than an hour after they got into the system. 

The queue never has more than 27,000 messages at the same time, and all together, the parallel modules process about
5000or 6000 messages per minute. So basically, the delay for a single message should never be more than a few minutes.
Evenif one module gets stuck, another will grab the next message from the queue. I believe the only way for a message
tobe stuck for so long would be for it to be grabbed by a module, and then for the database write to be somehow delayed
fora hour, although it's merely a simple insert performed with a prepared statement. 

The database in production is very busy with millions of writes per hour. Could there be a situation in which a
particularconnection gets "starved" while other connections are able to run queries without noticeable delay? 

How can I truck such locks down? Does anybody have any ideas other than starvation? The system lets me view statistics
ofhow many messages were processed in each modules and the average latency. None of the four modules running has long
averagelatency or low number of messages processes, so I don't think the issue is related to any particular thread in
my(Java) system being slow or locked away by the others. 

TIA,
Herouth