Syslog viewer can do what you are asking. Not sure if this cover all your needs. You can set a trigger threshold to only alert when 2 syslogs are seen for the pattern in a 15 min interval. In the other tabs you can specify the the sender and RegEx pattern. Does have multiple filters for each pattern accomplish what you are looking for?

Thanks,
Zak Kahl
Loop1 Systems
http://www.loop1systems.com