Outlook XP Auto complete

Hi chaps, one of our users has an intermittent problem.

when composing a new mail he types the recipient name and,, mostly,, after 3 letters it will auto complete (display a list) but sometime it doesn’t.

and i mean even using the same 3 letters it sometimes completes and sometimes just sits looking dumb?? Most of us could live with this but this user is our CEO and he aint got time to s”!7 – so please help.