I just ran into this issue and found a solution that fixed the issue completely for me, of course YMMV. It happened right after I decreased the game client resolution and I found that the GPU was now basically idle during gameplay (<10% utilization).
Going into the Nvidia App (Beta) and setting an application rule for pol.exe to "Prefer maximum performance" resolved the issue. Without that, I believe it was using adaptive power management to downclock the GPU multiple times per second between new frames from the client.
Setting max performance did not cause the GPU to stay pegged, as some online commenters suggested it would. Average utilization went from 8% to about 11% and the flicker is gone.
Hope this helps someone!