@Matt , Please capture a NetMon trace and filter the traffic for dllhost.exe process. I have seen this if there are frequent retries to perform RADIUS auth by RADIUS clients, such as VPN client.
If that is not the case, kill the process, restart the NPS service and capture 2-3 samples of ProcDump while the CPU is spiking up and if you need assistance with dump analysis, please open a support case with Microsoft.
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