Doug:

Since turning OpLocks back off, the client hasn't noticed a terrible decrease in speed...they do have a 2.2GB dataset, so waiting for a report isn't that unusual for them...lol!

They do have a new server with the latest Windows Server software (and all the updates), all the machines are Win7, the hardware and wiring are also new.

They only experienced the errors in the large dataset, two smaller datasets never exhibited any of the behaviour.

Such is life, I'm just glad to have found a/the solution and felt it important enough to share.