Outlook 2003 Delay

Hi
I have a DMVPN router at our data centre with 3 VPN sites connected.
The VPN-HUB router is connected to a ASA5520 firewall on the WAN interface (60), the Exchange server in located on the inside interface of the ASA (100) connected to a Cisco 3750 switch (Access Switch)
I have teamed up the NIC cards on the Exchange server and on the switch – the b/w is 2GB – NIC drivers on server are all up to date

Problem:
Outlook users are getting “Outlook is retrieving data from the server” – this happens at least 4-5 times a day also when the users create a new mail & go “to” to find a contact there is no global address list during this time.

Any ideas why this is happening?