I have similiar issue on my SRX300 which is running D60 software. I don't have high CPU utilization but my control plane is using 95 percent of memory:
Routing Engine status: Temperature 42 degrees C / 107 degrees F CPU temperature 56 degrees C / 132 degrees F Total memory 4096 MB Max 2908 MB used ( 71 percent) Control plane memory 2624 MB Max 2493 MB used ( 95 percent) Data plane memory 1472 MB Max 412 MB used ( 28 percent) 5 sec CPU utilization: User 9 percent Background 0 percent Kernel 4 percent Interrupt 0 percent Idle 87 percent Model RE-SRX300
I also have srx300 which are running almost the same traffic and configuration and are on D50 software and they don't have that problem.