Can't delete messages using delete key in Outlook 2007

After upgrading to Exchange 2010, we are seeing an intermittent problem where users in Outlook 2007 can't delete messages from their inbox using the delete key.  Opening the message and clicking delete from there will delete it from the inbox.  They can also drag the messages from the inbox to deleted items. 

The issue comes and goes during the day.  One minute delete will work and then it won't and then it will again.

Thanks,
Crystal