Thanks BOAC.
Anyone willing to help me understand it, as that was the thrust of my question? It would be nice to know what happens that causes it to malfunction in this way. I ask because I am shortly to become a trainer and understanding these things is important as I can't really just say to a new lad 'don't worry about understanding it'. I would like to be able to explain what is happening and why he should not put his blind faith in the VNAV profile (which many inexperienced pilots do on the line).
PP