OK, I seem to have resolved this issue. I was finding it impossible to work on my machine as it was hanging so often, or running at a snail's pace. I lost patience with trying the usual trouble-shooting remedies (repairing disk permissions, reseting PRAM, etc.), and with the more exotic hocus-pocus (running in 32-bit mode, etc.), so I gave up and simply created another user account.
And everything is running fine so far ... (I'll post back if problems surface.)
• Creating a new user account is simple. I'd suggest renaming your current account first; that way you can use your current login details for the new account. (Takes two minutes).
• Getting most of your information over to the new account is breeze if you sync with MobileMe. If you don't, sign-up for a free trial. (Takes ten minutes tops.)
• You don't need to reinstall programs, they are all their in the Applications folder. Just launch them.
• You'll need to bring other folders and files over to your new account via the Shared folder, another drive, or, easier still, iDisk.
Altogether it wasn't as arduous as I assumed it would be, and took less time to do then I was loosing in a day re-starting my machine.
Hope that helps. Good luck!