That does not sound like a familar problem.  I have a few questions for you:

Does this happen for incoming calls or outgoing?

If outgoing, how are you placing the call (i.e. typing in the number,
dialing from contact card, from call history, etc)?

Do the contacts having this problem show the name correctly in the contacts
application?

Do they show the name correctly in the call history application (assessible
from the dialer)?

Do the contacts having the problem share a number with any other contacts in
your collection (for instance, a general work number, or a home phone number
of a family where you have contacts for more than one member of the family)?

If you move a phone number from one of the affected contacts to a new
contact, do you still have the problem?

If you add a phone number to one of the affected contacts and use that to
place/recieve a call, do you still have the problem?

Sorry for all the questions, but it's the best chance I may be able to find
something out about the problem you are having :)
Thanks!