Messages stuck in outbox for up to 20mins

We are on a P1 Office 365 plan.  I have five users, all running Windows 7.  Four are running Outlook 2007 and one is running Outlook 2010.

All users are having the following issues:

Messages stay in the outbox for up to 20 minutes after sending.  Some delays in receiving emails.

We keep loosing connectivity to Exchange, we get messages in the bottom right hand corner of outlook that say “waiting to connect” and information notice "Connection to Microsoft Exchange has been lost",  “Connection to Microsoft Exchange has been restored”.

I have taken my laptop home and get the same issues there so it doesn’t appear to be a network issue at our end.

Four of us have iPhones running iOS5 and we don’t get the same delays sending and receiving emails for the exchange account from the same network.

The issue is not found when using the Outlook Web app.

The issues are much less evident or non-existent very early in the morning or at Weekends (when loads are presumably lower)

We are in Australia (my understanding is that the exchange server is in Asia).

If I turn off cached mode we do not get the issue.

I’ve looked at the community posts, but haven’t yet found a resolution.  I have worked through the kb article 2441551”Outlook performance is slow in the Office 365 environment” and taken the following steps:

1. Cached exchange mode is checked.
2. We are using the offline address book
3. We have downloaded the offline address book
4. We have checked the OST size (my mailbox is currently less than 200MB)
5. We have emptied all folders to reduce the size of critical items.
6. The only active add ins are Microsoft services
7. Safe mode does not improve performance
8. Lync online not installed
9. No email scanners running.  Anti-virus is Microsoft Security Essentials.  We experienced the same issues when using ESET.
10. We don’t use group policy (none of the default settings from 365 have been changed)
11. We’ve repaired outlook, but no improvement was found.
12. We do not have additional mailboxes to cache
13. We are not synchronizing sharepoint or RSS feeds.

Please help.