Well, considering I had to wait until Wednesday to be able to post the unit to The Netherlands, I have been having a little play and done some testing.
I have been playing with the settings on the NAS for a few days and I think I have found the problem and it was so simple! I said it probably was and I think I have solved this for good, though I may jinx it by saying this:
I did a RAID check and afterwards, I have had no more pausing and I have watched the movie Twister all the way through and no jumps or pauses. I am very relieved. There is one bonus, because the problem now appears to be resolved, I no longer have to return the unit to Buffalo in The Netherlands. What a result!
If anyone on this board has a similar/identical problem, run a RAID check on the entire volume installed, select "Array 1" to check. If this does not work, you may well have a faulty unit.
Thanks to everyone who assisted me in this difficult and frustrating time, I really thought I was going to lose all of my data!
Thanks again.
Thomas.