There are several possible reasons you are experiencing CPU throttling, overheating, and system lockups.Those include:
Using default settings in the BIOS for the CPU voltage (VCore) will usually set the CPU Voltage mode to Adaptive Voltage. Adaptive Voltage tends to add more voltage during stress testing than when using the CPU for normal purposes. Higher voltages will cause any CPU to become hotter.
Adjusting the CPU Core Voltage might help you reduce your CPU temperature, but that alone will most likely not be enough to fix your throttling and over heating issues.
Haswell type processors like yours tend to run hotter than other Intel processor generations. The boxed Intel stock CPU cooler is barely adequate in its ability to cool an i7-4790K Devil's Canyon CPU. This CPU requires a larger, after market CPU cooler IMO, and many PC enthusiasts would agree with me. But that is only part of the solution.
What are you using as a PC case? How many and what size cooling fans does it have? You must supply the CPU cooler with cool air from outside the PC case, and remove the heated air inside the CPU case from the CPU and other components. Most video cards exhaust some or all of their heated air into the PC case, which then becomes the air used to (try to) cool the CPU. The air temperature in your room may be 25C, but the air inside your PC case could be 35C or more. That makes the CPU cooler's job much more difficult, if not impossible. We really need all this information to diagnose your CPU temperature problem. Also, what model of video card are you using?
You can easily monitor the CPU Core Voltage (or really the VID) with IXTU. In the lower part of your screenshot, in the area with the Package Temperature, you can see a small blue wrench icon on the far right side. If you click on that, you'll get a list of many monitoring options, including Core Voltage. Check the Core Voltage entry and it will be displayed with the other items you have now. If you hover the mouse pointer over the Core Voltage entry, the maximum and minimum values will be displayed.
You can also add Core Voltage to the graph section on the lower left part of the screen. I suggest you add both.
You can change the Core Voltage Mode, and the Core Voltage itself in the Advanced Tuning section of IXTU. If you've never done this before, don't try without first learning about what you are doing.
You should post a screen shot of a stress test with Core Voltage monitoring added to the display. You should also post a screen shot of the Advanced Tuning, All Controls screen, so we can see what Core Voltage mode, etc, is currently being used.
Cooling an i7-4790K with the stock Intel CPU cooler while stress testing will be a challenge, sorry to say.
thanks a lot for your answer.
The case is a standard Midi Tower. With 2 x 120mm fans at the front and rear and a Gigabyte GTX 970 G1
I tried to reduce the Core Voltage, current max 1,2820V down to 1,1900V - 1,2000V but my PC restarts as soon as I start the stress test.
Current settings from Advanced Tuning:
And after stress test:
I think I've to get a better cooler and maybe a bigger tower. Really don't want to risk CPU's being damaged by 100°C