K so to set this up, i been having this issue since my 780 to where clock would stay at a high clock, not the full boost clock but a high clock. Right now my 980 classy is stuck at 1291, it boosts fully up to around 1525 but when it decides it wants to get stuck, its always at 1291 randomly. I have it on adaptive of course, i have chrome to not use gpu accel and everything else that uses that off but does not seem to matter in the sense that i dont even have to open them for it to randomly get stuck sometimes. 
 
It has to be something with my pc, im using a new mb also and the 780 i did have i sold to friend and he says he has never yet to this day had the issue and we use alot of the same programs and he has had card over 2 months now. I have this problem at least once a day and i normally reinstall gpu driver and fixes it but now that aint even working and i normally didnt mind reinstalling but now that i use geforce exp, i always have to redo all my folders for game locatons ( i have 5 internal HDs with games ) and takes a bit to setup. 
 
I normally pride myself in fixing software issues but this one has me stompped and cannot figure it out, googled around but nothing really solid. I use to use the .bat files to force idle and constant boost but meh..been awhile since ive set that up and honestly just forgot and rather not have to use that method again. 
 
So if someone has a fix or maybe a proper way i should reinstall the driver to fix the issue possibly as i hate to even have to reinstall AB as ive done alot of tweaking settings that i know i would have to redo, not counting gpu OCing as thats np but ya, anyone got a idea or solution :/ ? 

Sajin
Here are the top causes of this issue...

#1 You're running multiple monitors. Fix: Enable mutil display power saver using nvidia inspector.
#2 Your machine has a virus. Fix: Scan computer using malwarebytes.
#3 One or more programs running in the background are using your GPU. Fix: Perform a clean boot of windows.
#4 Power management mode inside the nvidia control panel is set to prefer maximum performance. Fix: Set power management mode to adaptive then reboot your computer.
#5 Running a resolution of 2560x1440 @ 144Hz. Fix: Change refresh rate to 120Hz.
#6 K-Boost enabled inside EVGA Precision X. Fix: Disable K-Boost.

1# Using single 1080 monitor 60hz
 
2# Ran my normal scan last night ( use several programs, none are running at same time of course ) and all was clean.
 
3# I can close everything out that i assume is causing it and still a no go, i dont install alot of stuff that is required to always run or on startup but i have checked and nothing that is running should cause it to stay stuck but i am gonna recheck this part again though after this message.
 
4# Set to adaptive
 
5# Running 1920x1080 at 60hz
 
6# Running Afterburner but issue is also in precision with no Kboost enabled. 
 
 
Its def weird issue and friend uses AB and ya, i fixed the issue as always with a fresh install of driver and if i click Clean Install it fixes it till it messes up again, if i dont click Clean install a fresh install wont work. 

Sajin
It may be a driver issue then, try going back to 347.09

 
ya forgot to mention i tried like ugh idk how many different drivers and issue always arose. Like i was having this issue since i ran my 780 so its something on my end i know just not sure what. What is that 1 method/program ppl always recommend to uninstall a driver? Idr name but i know it requires like a uninstall then u boot into safe mode then run program to remove left over files or something. I know alot of ppl say its not needed but maybe thats what i need to fix it? 

DDU is the program you seek. You said you're now running geforce experience... What happens when you uninstall it from your system and reboot the pc?

Sigh....and it happened again. It was working fine, browsing web, useing IE/chrome among other programs, ran a few benches and it downclocked fine. Just started playing AC unity which i noticed something strange but ill get to that at end. So closed out AC and it stayed at 1291...Now there is a program i use that lets me use my ps4 controller on pc through bluetooth. Noticed before itself made it go to 1291 and would sometimes idle back down to 135 where as other times i would have to close it out to make it idle back down so ugh idk...guess ima have to do the .bat trick again >.>
 
Anyway onto the strange part, before i uninstalled AB and the gpu drivers, i screenshotted my AB OC settings. So once everything was reinstalled i just copy them back, so i did of course. While in game my gpu boost clock now was 1551, instead of 1521 which was the max i saw during AC before and benches. Not complaining but im sure that wont be stable just weird why its going that high now when its on the exact same settings as before but it never did crash so maybe stable idk but ya, still got the bug, will do the .bats tomorrow i guess, thanks for the info and help anyway :p 
 
 
EDIT: Ok so before i logged off pc, i went through task manager to see what was running, saw several nvidia stuff running that im assuming is tied with gforce exp. So started randomly just End Tasking them and glanced over and saw it idle back down to 135 >.> so think i found the issue for it doing it this time as to before idk...as i never used gforce exp but was like 4 nvidia things so maybe wasnt all gforce related but ya, will test some more tomorrow to see which one is the culprit and report back. 

-TheFalleNOne-
Strange , i got the same problem with my gtx 970!It wont go into idle and downcloack itself , always at 1215Mhz at 60 degrees.

You wouldn't be running dual monitors would you?
 
Andddddddddddd to keep this OT for the OP maybe start a thread for your issues? Gets confusing for the OP and those trying to help with the issue at hand for hopeful diagnosis of the issue.
 
Also see post #2 in this thread and see if any of Sajins procedures help to resolve your issue.

I knew it was related to geforce experience. The newer driver must have added some performance improvements that is why you are seeing a higher boost now than before.