This is a known bug with Dell PowerEdge T320 Servers and Server 2012 R2. In my case, it was happening randomly only once or twice a month. The CPU would go down to below 0.20 Ghz and the server would crawl.
If your CPU has been tested and is not overheating, this is the workaround:
In the PowerEdge Bios, change the System Profile to Performance Optimized. This will disable C-States and C1E.
Mine was previously set to Performance Per watt (OS).
I haven't tested it, but I think the issue also occurs with Performance per watt (DAPC)
Dell, how about fixing your bios?
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Log Name: System
Source: Microsoft-Windows-Kernel-Processor-Power
Event ID: 37
Level: Warning
Description:
The speed of processor 0 in group 0 is being limited by system firmware. The processor has been in this reduced performance state for 71 seconds since the last report.