A BIOS update is available. The release notes state:  "Updated Intel Management Engine (ME) binary to Rev 2.1.5. 0x8B, to address issues that resulted in unexpected temporary or permanent Processor frequency degradation."  I tested the bios and it fixed the issue described in this thread.   ============================================= Dell Server BIOS PowerEdge T320 Version 2.4.2       T320_BIOS_8H9VP_WN64_2.4.2.EXE | Update Package for Microsoft® Windows® 64-Bit. (22 MB)BIOSRelease date 07 Apr 2015Last Updated 07 Apr 2015  Recommended Version 2.4.2,2.4.2  Dell Server BIOS PowerEdge T320 Version 2.4.2   I have not tested this bios to see if it fixed the throttling.    
T320 with Xeon E5-1410 will throttle and get stuck at low speed.  Cold rebooting server will fix.  Random issue occurred on 2 servers now.   One was set in the bios to DAPC.  The other, Windows controlled (yes Windows power settings were set to Performance).  One had the latest bios (Server 2012 R2) and the other had an older bios (Server 2008 R2).   I stress tested the cpu on both and observed no overheating.This is a known bug with Dell PowerEdge T320 Servers with a Intel Xeon E5-1410 v2.  On the Server 2012 R2 server, it was happening randomly only once or twice a month.  On the Server 2008 R2 server it only happened a couple of times at random.  The CPU would go down to below 0.20 Ghz and the 2012 server would crawl and service availability was seriously degraded.   This also happened to another client T320 Server with Server 2008 R2 with an Intel Xeon E5-1410.  Multiplier would not increase from 12 and the server was running at ~1.2ghz.  If your CPU has been tested and is not overheating, this is the workaround:  Resolution:  In the PowerEdge Bios, change the System Profile to Performance Optimized.  This will disable C-States and C1E.   Instructions:  www.ponjavic.com/.../disable-dell-poweredge-m620-cpu-throttling This has to be a bug in the bios or the microcode of the CPU.  The above resolution works.My clients don't care about the minimal loss in power savings.  But all T320s ship with the default set to Performance Per Watt (DAPC).  This is a serious issue. DELL:  This problem is hard to duplicate, but needs to be fixed. I'm surprised it hasn't been resolved already.
PowerEdge T320 CPU Throttling - SERIOUS BUG