Ours is set to "128m" - which is the default for CFMX 7.0.2 I
believe.
Was 512m too large for your environment perhaps?
I would try adding the setting back into the JVM args, and
changing it to 128m to see if that works.
Also, be very aware of extra newline or whitespace characters
in your JVM arg list. It is easy to get confused if you're editing
the args via the <textarea> provided by the JRun console. I
personally prefer to manually edit the jvm.config file, located
here:
.....\JRun4\bin\jvm.config