what os? what java? how many chunks / downloads?
you can try to start jd with the follwing command and see if it helps
java -Xmx512m -Xms64mb -XX:+UseConcMarkSweepGC -XX:MinHeapFreeRatio=0 -XX:MaxHeapFreeRatio=0 -jar JDownloader.jar
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JD-Dev & Server-Admin
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