My guess is that you are running out of perm gen space to load classes. When that happens and no classes can be unloaded, the JVM will start performing full garbage collections repetitively which will prevent your application threads from getting any processing time.
Go into the JVM options tab (Application Server --> JVM Settings --> JVM Options) and
bump up the "-XX:MaxPermSize=192m" setting to something bigger (such as 512m).
Alternatively you can add the following four new JVM options to write garbage collection data to a log file to see if in fact full GCs are being performed repetitively:
-verbose:gc
-XX:+PrintGCTimeStamps
-XX:+PrintGCDetails
-Xloggc:${com.sun.aas.instanceRoot}/logs/gc.log
Joe
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