transaction blocking on COMMIT

I am seeing a strange issue on a database using jdbc. Regularly, 4 or
5 times a day, I see something like a "stutter", where a bundle of
maybe 30 transactions suddenly finish at the same time. It looks like
(it is quite hard to catch this exactly) that the lead transaction
which has been blocking the rest has been blocked in COMMIT. In each
case it blocks for almost exactly 30s, just over, and once it goes
through, releases locks, and the others clear behind it.

My question: what are the range of possibilities that might cause a
COMMIT to block? I haven't seen this before. Is there anything
suspicious about the regular 30s? Occasionally we see 60s, which
seems likely to be two sets of 30.

Regards
Bob