Thanks for looking into this again, Michael.
I just logged into phpMyAdmin to check the MySQL version and noticed this warning message at the bottom:
"Your PHP MySQL library version 4.1.22 differs from your MySQL server version 5.1.48. This may cause unpredictable behavior."
This looks interesting. I'll follow this up with my host. I have a dedicated server for this particular site, so I should be able to get this altered. Will post here to let you know how it went.
Again, thank you so much for your help. I have advised my client that they should send you a donation and they agreed, so hopefully that happens. Not that donations ever amount to anything approaching a slim fraction the time you put in.