Exchange Task - Move mailbox from 5.5 to 2003

[Lukey]

I have a mixed mode environment and have installed a separate Exchange 2003 server. This was all running fine and I had migrated about half of the mailboxes over from the old 5.5 server. When I go to move a mailbox now, it allows me to do it but take ages. It seems to stick on 10% regularly but it takes about 1 hour to transfer even a small mailbox (100mb).
Previously, this took about 5-10 mins per mailbox. Normal data transfer between the servers is fine but I have no idea what could have happened.
Any help would be greatly appreciated otherwise I will be migrating the mailboxes over for the rest of my life !!