Let me preface the following with the fact that this is the first pc I have built in the past 20 years.  I think my last build might have been with a Nehalem processor.  So, there is a good chance that I may be just missing something obvious.
The build consists of the x299 Dark, i9-7940x, EVGA T2 850 watt ps, Samsung EVO 960 pci M.2 SSD and Nvidia Titan xp gpu.
Both cpu power cables are connected to the x299.
I have not changed anything in the Bios other than what was necessary to install the EVO 960.  Bios at build was 1.04.  Windows 10 pro 64 installed without any problems.  To date I have power cycled the machine 16 or more times with  a total runtime of more than 100 hours.  It has been stable.
Using the Evga Eleet software, cpu temps at idle - 3.1 GHz- has been 30C.  Temps across all 14 cores range from 29C to 31C.  The cpu speed was taken from CPU-Z and the Performance section of Task Manager.  The Eleet software appears to only statically list the lowest multiplier.
I have run several iterations of Prime95 for 1, 3, and 5 hours over the past 3 weeks.  Temps range from 51C to 54C for cpu package and across all cores.
I recently downloaded and installed Intel's XTU.  I used this only to monitor temps, speed and core utilization.  When Prime95 was run, speed dropped to 3.5GHz and "Current Limit Throttling" lit up.  All temps were still in the 51C range and the XTU program reported a power dissipation of 26 watts.  I have no clue despite searching for days, what is causing this.
So, I thought I would do the 1.07 Bios update.  That went OK and again the only Bios changes I made were to enable the EVO 960 SSD.  I've made no changes with Eleet or XTU software.  I uninstalled the XTU software prior to the Bios update.
Anyway, after the Bios update the idle speed, as shown by CPU-Z and the Performance tab in Task Manager, now fluctuates constantly.  It goes from 1200MHz to 4.3GHz rapidly, along with changing cpu voltage.
I could understand some small changes occurring slowly, but this rapid "flopping" doesn't seem right.
Running Prime95, CPU-Z reports a maximum speed of 3.8GHz.  I assume this is a core 0 speed and should hit 4.4GHz, if you believe Intel's spec.
 
Yeah, I know this is a lot, but I'm really not sure what to try next.  Any help would be appreciated.
 

Welcome to the EVGA Forums FuzzyOnion
 
in addition to bdary's great post above
 
In Win 10 pull the network connection
W10 tends to update all the time, depending on your options & if you disabled all the stock Apps - Especially Xbox DVR 
do a "Clean Boot" & then close all background software


 Basic troubleshooting steps. Windows clean boot, clean driver install, debug mode, etc
  

Also, power plan in Windows 10 to High performance, will have it run stable. Be advised though, that if you set AVX offset to 0 and you put OC on it, it will get hotter because AVX operations are heavier on the usage side than non AVX. Also Mesh comes into consideration in regards of power consumption and the likes.
 
Intel also uses the coolest core to put the max boost clock to it, which is not always the Core 0 ... For that you can use HWinfo or CoreTemp to read that.