Import Contacts from a Saved File is Limited to Only 5 Contacts

I have 30 contacts in a saved file (Contact List.ctt)
In Windows Messenger 5.1.0706, I click on File and then Import Contacts from
a Saved File.
I select the saved file (Contact List.ctt) and click Open
A message is displayed indicating that "The following 30 contact(s) will be
added to your contact list."
I click "Yes."

Five (5) of the 30 are added to the list. Only 5! arrgghh!

I have recreated this "problem" multiple times on different PC's.

Several minutes later (3 to 5) I can repeat the above steps and 5 more
contacts are added, but I MUST wait. If I try again too soon no contacts will
be imported.

In the past, all 30 contacts were imported together. I am suspicious that
something in Windows XP Service Pack 3 is causing this new behavior, but I
have not yet been able to confirm.

Does anyone know how to remove this 5-at-a-time limit on importing contacts
from a Save File?