AP have you run the Heaven Benchmark? I would let it run for a long time then hit Benchmark and see what you get. Like I told you I really think a 550W power supply is really underpowered. It is possible the GPU voltages might be fluctuating. Doing a lot of searching when I had the 970 leads me to believe there is not nearly as many 980 problems out there. CaptJack has never complained about stutters that I am aware of. I would be willing to bet he has a hefty power supply in his new system. as for ASIC that is like debating religion. Lots of opinions and not many consensus as to what it really determines.
I would go into the bios and disable the on board sound card and maybe even the network card. Then try it with no sound (no USB headphones either) offline and see if you still get the stutters. Narrow it down to the basics.
Another thing after the bios upgrade and the other things I mentioned would be to do a Nvidia driver cleaner. Also did you run the Resource Monitor and check to see what your CPU, drives and network are doing when it stutters? There could easily be something else running in the background causing it.
Also try this
Go to Nvidia Control panel and set Power management mode to use prefer maximum performance and change maximum pre rendered frames to 3.
Did you unpark the CPU cores?
Use Geforce Experience and place the card in Kboost mode