Keep in mind that you do not need to worry about the Cores temperature, you need to pay attention only to the CPU temperature (which is not the same thing).
The maximum recommended temperature for this CPU Is 67.9
As long as the CPU temperature does not reaches above 67.9 degrees Celsius, the processor will be running under thermal specifications.
You can double check the maximum recommended temperature for this processor at:
What is the failure message are you getting?
If you are running the default settings on the BIOS and in the Intel Extreme Tuning Utility (Intel XTU), I suggest to get the latest BIOS version for the DX58OG and run the test again. We may be facing a problem on the BIOS version that is causing this error. You can get the latest version (0817) from:
the error message says "The CPU Test failed due to thermal throttling" (see attached pic.). I have upgraded to the latest BIOS, verson 0817. Maybe there is something wrong with the XTU itself. Prime95 is running w/o problems for an extended time. When running the XTU stress test, there are always small dips in CPU utilization but never any CPU throttling in the graph.
Tcase is the temperature of the CPU packing or case, we could say is temperature around the processor. Could you give it a try and use the processor diagnostic tool and see if its giving you any errors or if the Thermal Throttling problems is isolate to the Intel XTU*
This is the link for the Intel processor diagnostic tool* is: