This client has access to several calendars and 2 other mailboxes, but Outlook takes up to 5 minutes to go from one calandar to another or one Inbox to another, if anything else is selected while outlook is doing this then the "Outlook is not responding" error comes up.  Client only uses up to 3 calandars at a time and wouldn't have any other programs open. PC has 512MB RAM. no other problems with other programs.
Outlook is on cashe mode, this can't be changed as it is the way the office works.  client doesn't have a PDA attached.
Have tried deleting Outlook profile, reparing outlook and tried disabling add-in's, but nothing has worked, how do I fix this issue.