Looking for "gaps" (from a time perspective) in data

This is on 10gR2 standard edition.

We have a table defined like this:

txn_id
date_start
date_end
url_called

which contains a record for every time our application processes a request from a browser. We would like to be able to spot any "gaps" in the data - which would indicate a period when the service was unavailable for whatever reason.

To do this we need to do a comparison between the date_start of each row and the one that preceded (or followed) it and then see if the period of time is greater than a to-be-determined threshold.

Can a single SQL statement be constructed which could achieve this? if so could you point me in the right direction? For those who haven't followed what I was asking about, if you are in the majority then please accept my apologies for not being more concise :)

regards,

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jeremy