In case that is your issue - the only workaround at this point - is posted at the following location:
http://blogs.technet.com/operationsmgr/archive/2009/10/07/when-taking-agent-out-of-maintenance-mode-old-events-are-reprocessed-and-new-alerts-generated.aspx
Resolution: There are three potential workarounds for this issue:
1. Clear the event log prior to putting a machine into Maintenance Mode.
2. Manually setup overrides to ignore the events.
3. Add the following registry key:
HKLM\Software\Microsoft\Microsoft Operations Manager\3.0\Modules\Global\NT Event Log DS
Create a DWord value named MaxEventBufferSize and set it to a decimal value of 500000.