Install a Monitoring software and see if your Processor is overheating. If the temperature is going above 57c then it is overheating.
You can check in BIOS to see what temperature your processor is running at that time. It should be almost the same as running in idle so it should have a low temperature.
With 6 cores enabled, download and run OCCT CPU,GPU, and PSU tests. For the Global Temperature in OCCT set it at 58c. That way as soon as the processor reaches 58c the test will shut down automatically.
While testing with 6 cores keep a close eye on your Temperatures and PSU outputs. The PSU Outputs must be within 5% tolerance of 12/5/3.3 vdc.
If your processor starts going over 57c while under stress than you need to look at your CPU Cooler. It may not have been installed correctly or tightly after replacing the motherboard.
Otherwise it could just be a defective processor.
NOTE: TDP Ratings is the amount of heat the processor generates for the CPU Cooler to remove. So you will need a CPU Cooler rated at least 250 TDP wattage to keep your processor cool under heavy loads.
I had a FX 8350 with a TDP rating of 125 watts. I needed to install a Air CPU Cooler rated at 150 TDP watts with 2 High CFM fans to keep the processor from overheating under heavy loads.