Hi MRBULL,
I happen to have the exact same problem, but I believe to have managed to find the cause.
You need to set the refresh rate of both monitors to the same value, otherwise this form of stuttering occurs (I don't think that this is working as intended, but it fixes the problem).
Also if you are using a SLI configuration I found that the NVidia drivers become very picky.
What did the trick for me was setting both monitors to 60Hz, remove SLI, disable GSync, re-enable SLI, set both monitors (primary first, then secondary) to 165Hz and re-enable GSync with the windowed-mode option - then the stuttering should be gone.
As I have two PG279Qs your problem may not be caused by the same issue, but anyways I hope it helps.