Losing exchange contacts

Hi,

I have an iPhone 4S running 6.0.1 that keeps losing a large majority of the sync of contacts with our 2003 Exchange Server.

It will randomly 'lose' contacts, but then the following day, they will all come back again. I have tried deleting and re-adding the exchange account on the phone in the past, but the problem has come back again! It seems to be more the older contacts, as the more recent ones added to exchange seem to be staying there. Either way, it is not all of them, but a signicant amount to be causing a problem.

Has anyone else had this issue or know what might be causing it?

Thanks