Delay when typing in the address fields

When I open a new message in Outlook 2007, there is a substantial delay
between keystroking letters and having the letters and the auto suggest drop
down appear in any of the address fields (To, CC, BCC. It is usually at
least 6-8 seconds if nothing but Outlook is open (main window and new message
window only) and up to 20-25 seconds if numerous windows are open.

The lag is not present when typing in the Subject line or main message field.

I've tried all sorts of stuff, like changing message format (plain text,
rich text, html), turning off the local copy of the address book, tweaking
options and nothing changes the lag.

I'm running Outlook 2007/XP on a Gateway 3.2 Ghz machine with 2 GB of ram.
I know I'm update to on service packs, hotfixes, etc. This is in a setting
with an Exchange server, but other computers on the system do not display the
same behavior.

Thanks for any suggestions on what else to try to fix the problem.