Hi,
i suppose you are usin sun's Hotspot JVM
GC :
Add the folowing properties to your weblogic JVM and restart it :
-verbosegc -XX:+PrintGCDetails
Heap Dump :
Add the folowing property to your weblogic JVM and restart it -XX:+HeapDumpOnOutOfMemoryError
Thread Dump :
Using the weblogic server administration console, go to "Environment/Servers/your server/Monitoring/Threads" and click the "Dump Thread Stack" button
Emmanuel