Thanks all for your input...
Jack W and Johanbgoot....
I have been using Yasu but OnyX does appear more versatile so thanks for the tip.
I have tried rebuilding the Entourage database but no luck there.
Paddy....
Thanks for the links, I'll work my way through the suggestions there.
Krissel....
No, I don't have the tools you mentioned but perhaps I should invest in one of them?
I've run Disk Utility.
Archive and install of the OS may well eventually be the way I'll have to go.
I'm hanging out for one of the new iMacs that are rumoured to be coming soon. I plan to reformat the drive on this iMac as soon as I have the new machine. Pity this computer couldn't have remained trouble free for just a little longer...