We had an issue earlier this month with the Win 10 App Readiness service (we're also on 1803) going crazy for an unknown reason.  There are lots of articles out there with similar complaints and the solution for us was to disable the service.  So far, we have not seen any negative impacts of doing so.  It sounds like you're not able to get into a VM to see what app/service is spiking the CPU but wanted to mention it as a possible starting point.