To validate that you are using stable perf API
run
"C:\Program Files (x86)\checkmk\service\check_mk_agent.exe" section wmi_cpuload
You should get relatively short and clean output:
<<<wmi_cpuload:sep(124)>>>
[system_perf]
Name|ProcessorQueueLength|Timestamp_PerfTime|Frequency_PerfTime|WMIStatus
|0|961086594870|10000000|OK
[computer_system]
Name|NumberOfLogicalProcessors|NumberOfProcessors|WMIStatus
MY_PC_NAME|12|1|OK
In this case, you should not have problems with CPU-load monitoring. Still do not hesitate to infrom checkmk if problem persists.
Sincerely
SK