Exchange 2010 SP1 free/busy issues

I have a couple of users who are using Outlook 2007 who occasionally have issues seeing free busy of other users. We recently did a migration from Exchange 2003 to 2010 SP1. Prior to the migration, there were no issues. Now, all mailboxes are on Exchange 2010 and certain users have occasional issues. These users are administrative users, who schedule meetings more than most (which may mean the issue is not specific to them but they just use calendaring more often so they see it more often). I've seen cases where they will pull up the free busy information of three or four users and all of them come back successfully except for one, which gives the hash marks indicating free busy is unavailable. This only happens occasionally. It is for users who are on our corporate network, so they have a good network connection.

Ideas I've come up with are:
1. Have users whose free busy is not showing up run outlook /cleanfreebusy
2. Turn up event logging on the Exchange server. Should I just set it to "expert" for the availability service.

Questions:
1. I'm not seeing any additional event logs for availability service or free busy after turning up the logging level. I have done an IISRESET on both of the CAS serers and restarted the mailbox assistants service on both of the mailbox servers. Any thoughts as to why?
2. What else do we need to do to figure this out? Any other troubleshooting steps I can take?
Any other suggestions?