In order to troubleshoot a problem with an Event Collector throwing a GC Overhead Limit Exceeded message you must know what can cause it.

For a concurrent collector, if more that 98% of the total time is spent
in garbage collection and less than 2% of the heap is recovered, an
OutOfMemoryError will be thrown. 

The policy is the same as that in the parallel collector, except that time spent
performing concurrent collections is not counted toward the 98% time limit.  
In other words, only collections performed while the application is stopped
count toward excessive GC time.  Such collections are typically due to a
concurrent mode failure or an explicit collection request (e.g. a call to System
.gc())."

Tuning the Symantec Event Agent will resolve most if not all instances of this.

For information on Tuning the Event Agent, read the article Symantec Event Agent Tuning Information