There's no reason that TB wouldn't support more addresses than a mere 300. I've got 1079 addresses in my AB and never experienced the problems you describe.

The only thing I can think of is that you either have no writing rights for the AB (restored from CD or it's located in a directory where you cannot alter files) or it has gone corrupt.

To fix the first is an exercise in Windows, to fix the second I'd suggest to create a new address book and copy all addresses from the old AB to the new one.
(Open the AB
File -> New Address Book
the rest should speak for itself)

What's the physical size of your AB?
How did you copy your old addresses to your new AB?
My 1080 addresses containg AB is 348 kB (and encrypted) while the last copy before I switched to On The Fly Encryption (pro version only) was 899 kB and contained a mere 922 addresses. Those differences come from the fact that TB doesn't compress its AB when you delete items, so there's a lot (four years)of deleted addresses in my old unencrypted AB.

A good way to delete that clutter would be to create an internal backup with TB, AB only, and restore that backup.