"Type a contact to find" seach box doesn't work properly

I have a user who is using Outlook 2002 SP-2. He has over 1500 contacts in his Contacts folder. I exported his contacts to a .pst file and imported the file to my computer for testing.

Here's the problem: Most of the time when I type in a name in the search box it returns the appropriate results. But sometimes when I type in a name it doesn't find any of the contacts, even though they're listed right there in front of me!

For example, I can search for "Mark" and it brings up the "Choose Contact" window and lists 15 different people with the name "Mark" (which is the desired result I want). However, if I search for "Mike" it brings up the "Check Names" window and lists all the Mikes in the Global Address List. It doesn't find any of the 40+ Mikes in the Contacs folder.

I tried this on two different computers, one running Outlook 2002 and one running Outlook 2003 and I get the same problem.

Any help with this would be greatly appreciated!! Thanks.