Runnaway UPDATE Statement In SQL2005 - How To Resolve?

I am in the process of moving a SQL2000 database to a SQL2005 database.

Porting from: SQL200, Windows Server 2000(SP4) (32 bit dual processor 4GB RAM)
to:SQL2005, Windows Server 2003(SP1) (x64 bit dual processor 4GB RAM)



After porting the database from SQL2000 to SQL2005 (no changes)
I then compare an update statement running from Management Studio on the 2003 Server and and Query analiser on the 2000

Server.

SQL2000 completes the command in 2 minutes SQL2005 is still running after 60 minutes.

SQL2000 is the live/production system with users connected, the SQL2005 is in a test environment with no other processors

running.

The SQL2005 activity monitor shows:
-----------------------------------
3 suspended processes in CXPACKET wait state and
2 runnable process high CPU counts (SQLServer running at 100% cpu).
All processes with the same Process ID.
Wait time is 0
High CPU counts for the runnable processes.
Low physical IO
No lock conflicts


When I add the "option (maxdop 1)" to the update statment then:

The activity monitor shows:
---------------------------
1 runnable process with a high CPU count (SQLServer running at 50% cpu).
Wait time is 0
High CPU count for the runnable processe.
Low physical IO
No lock conflicts

How do I debug this situation?
Why is the SQL2005 unable to complete the task?

The update statement is as follows...
-------------------------------------

update BI_LENDING_TRANSACTIONS
set [Balance Movement Month] = M.[Balance Movement Month]
from BI_LENDING_TRANSACTIONS as T,
BI_BALANCE_MOVEMENT_DATES as M,
BI_COMPANIES as C
where (T.[Transaction Date] >=
(SELECT DATEADD(d, - 70, minDate) from (select min([Transaction Date]) minDate
from p_BI_LENDING_TRANSACTIONS) t1)
OR
T.[Transaction Date] >= C.[MostRecentSnapShotDate] or
T.[Value Date] = T.[Balance Movement Month] ) and
T.[Value Date] <= C.[MostRecentSnapShotDate] and
T.[Value Date] >= T.[Transaction Date] and
T.[Company_Code] = M.[Company_Code] and
T.[Value Date] > M.[SnapShotFromDate] and
T.[Value Date] <= M.[SnapShotToDate] and
C.[Company_Code] = M.[Company_Code]