If 600,000K of memory means 600,000Kb of memory it might be OK because the default maximum memory jD can use is 512MB. You can increase this if you have a very large number of links in your download queue.
The startup command for jD is :
Code:
java -Xmx512m -Dsun.java2d.d3d=false -jar JDownloader.jar
and you can increase the value for the -X parameter.
My jD only uses a few percentages of my old CPU. 50% isn't normal unless you've a PC dating of the previous century.
You're using an unstable version of the JRE. I would revert to update 21 ("**External links are only visible to Support Staff**).
If your problem still happens I would provide
a complete and detailed log.