When I'm idle (I'm stuck on the desktop) after 5 minutes, I can see a high CPU use from the system process. See the following screen from Process Explorer as an example.
As you can see I'm running Windows 10.
How can I fix the problem?
you likely have the idle maintenance scheduled task that runs and scans your RAM for error. Check my answer and look for ntoskrnl.exe!RtlpGenericRandomPatternWorker, ntoskrnl.exe!RtlpTestMemoryRandomUp calls in callstack of system process.
looking at the picture shows exactly what I told you. I see the ntoskrnl.exe!RtlpGenericRandomPatternWorker call, so the memory test during idle maintenance. so disable this task to fix it