I have the same problem (srx210h) and my flash storage is only at 20%, so it's not a problem with full storage. It happened when I downgraded from 10R1.8 to 9.6R1.13. Which I did because 10 gave me too many issues with the JWeb interface, ironically.
I reupdated to 10R1.8 and now I often get a control-cpu locked to 100% (the device has practically no traffic).
It often give me some of the following errors on JWeb :
1. Access Error: 404 -- Not Found (configure)
2. Data failed to initialize (dashboard)
3. Data request failed status: 503 (dashboard log window)
4. Your session has expired (with only a few second of inactivity)
5. chassis view doesn't work and is blank
This is intermitent, but I see a corellation with the CPU at 100% when a problem happens.
The only thing that is constant is the chassis view which never displays anything.