i7 8700k problems

That's not particularly alarming. Also, STAY AWAY from that Overclocking software; it will lead to nothing but system instability down the line. When was the screenshot taken? Even with that one core being used a bit, your background usage is 13%, which isn't too bad (don't know what background tasks your running.) Do you experience any stuttering in game at all?
 


Hello. The screenshot was taken just now. Only windows applications were running in task manager. ( https://fil.email/CusT8wxG ).
About the stuttering... That is actually my problem... Whenever I'm entering any game or anything, i'm having huge fps drops.
 


Intel I7-8700K,
GTX 1070 Seahawk ek x,
Water cooling EKWB Slim 360,
Motherboard Asus ROG STRIX Z370 G Gaming Wi fi AC,
Corsair Vengance 16GB DDR4.

I've already reinstalled my windows twice only today. With different versions of it. Same problem.
I've also tried a benchmark. Here's the result: http://www.userbenchmark.com/UserRun/9830558 . After all that, it says at the bottom ' Very high background CPU (75%). High background CPU reduces benchmark accuracy. Find active processes with Windows task manager (CTRL+SHIFT+ESC). '
+ quick little side note: the temperature of the I7 is ~40-50C.
 
Have you applied ANY type of overclock?

All of this would point to a bad OC. I would recommend resetting your settings in AI Suite 3, then uninstalling it along with any other CPU OC'ing software you have. Nothing can beat the good ol' fashioned BIOS/UEFI OC.

Are you getting that temperature when idling, or during the test?
 


I've tried to apply OC on my proccessor. I think that's actually the problem. One core has stuck (not sure about this).
If I plug the pc out and wait some time, the core seems to be working. After I open an application/game, it's starting to use all of the core. ONLY ONE. If I only shut down the pc and turn it back on, the processor is still stuttering.
I've also tried to reset the processor with AI Suite 3 and uninstall everything that had to do with OC.
The temperature, when i'm benchmarking the processor, is between 50C to 66C(max.).

p.s.: When i open an application, every core works. The thing is that one core is used 90-100% and it stucks at that percent...