A recent .Net frame work update is causing the AAD Connect server to crash or the CPU utilization goes up to 100%

Go to Task manager and check the list of processes,

If MIcrosoft.Online.Reporting.MonitoringAgent.Startup is consuming high CPU or

If you have the following update,

KB4338420	Windows Server 2008
KB4338606	Windows Server 2008 R2
KB4054542	Windows Server 2012
KB4054566	Windows Server 2012 R2
KB4054590 KB4338814 KB4338419 KB4338605 KB4345418	General

Then update your Azure AD Connect Health Agent as soon as possible to avoid or stop a major issue in your infrastructure.

Use following link to know how to update them,

https://docs.microsoft.com/en-gb/azure/active-directory/connect-health/active-directory-aadconnect-health-agent-install#download-and-install-the-azure-ad-connect-health-agent

Reference: https://support.microsoft.com/en-gb/help/4346822/high-cpu-issue-in-azure-active-directory-connect-health-for-sync