Commit gets locked until vacuum is performed

Hi all,

I'm using JDBC driver 8.1-413.jdbc3 to access a PostgreSQL 8.1.3
database on windows.
Sometimes, when my application performs a commit of a transaction with
maybe 100 or 200 inserts, the operation gets locked, with no error
message or exception.
When that happens, if I perform a vacuum over the database (for example
from pgAdmin), the operation unlocks and the commit finishes correctly.

Any idea of what can be happening?

Thanks in advance.

Xavier