Blocking SPID ... No Apparent Reason

Hi,
I hava a JAVA application that updates a SQL2000 (SP3a)database.
The application handles different types of "jobs" which effectively update the DB.

One job in particular appears to block all subsequent jobs. It comprises of a large amount of inserts/updates in a single transaction. This is necessary as it is an "all or nothing" scenario - so we cannot break the transaction into smaller ones. The transaction appears to succeed as it reaches the COMMIT TRAN statement without error.
However the records do not get written to the database.
EM indicates a large number of locks held on the tables accessed by the transaction and these do not get released.

Using the SP sp_blocker_pss80, the blocking SPID has a waittime of 0 and a waittype of 0x0000 - the lastwaittype is WRITELOG and its status is AWAITING COMMAND

I am using MS SQLSERVER JDBC Driver SP2 (considering using jTDS)

I have tried
- increasing Transaction Log size
- Moving Transaction Log to a separate Disk
- Reducing Isolation Mode to Read Uncommitted
- Set AutoCOMMIT to true
- set Close Cursor on COMMIT
- set SelectMethod to Direct - (we use Cursor by default)

None of these have succeeded in fixing the issue.

The job will succeed if it is the first/only job to access the database.
But if another job precedes it - then the blocking occurs.
I have verified that the preceding job only holds shared dataabase locks
before the blocking job is run.

Each job will use its own JDBC connections to access the database for reading
purposes, but all of the writing goes through the blocking SPID.

Any ideas?
Thanks, Liam