The following KB article was released May 4 2006.
See http://support.microsoft.com/?id=914835 for full details
Under certain conditions, Microsoft Operations Manager (MOM) 2005 may stop processing incoming Simple Network Management Protocol (SNMP) traps. This generally occurs after MOM 2005 receives many SNMP traps in a short time. By restarting the MOM service (MOMService.exe), you can bring SNMP trap processing back online for a short time. However, the problem will probably occur again.
This problem occurs because MOM 2005 does not correctly handle a specific error condition that Windows Management Instrumentation (WMI) returns. This hotfix makes sure that MOM 2005 re-subscribes to the SNMP trap events with WMI after error conditions such as this occur.
A hotfix is available from Microsoft Product Support Services for customers who may be experiencing this problem.