The whole purpose of G-Sync is to control the display so a frame isn't drawn until the GPU has finished computing and rendering all the onscreen objects. Specifically, it will adjust (lower and raise) your fps on the fly as needed so the GPU and monitor are always in sync, and if the GPU is overloaded enough (moments when it can't even sustain 25-30fps) then you'll perceive the effect as frustrating sudden slowdowns or intermittent motion stutter.
Without NVidia G-Sync you'd default to good old V-Sync. With V-Sync, you'd be locked at a (generally) maximal frame rate but (when the GPU slows down from overload) the frames wouldn't always be fully rendered before display, so you'll perceive nasty flickers and tearing and other unwholesome partially-rendered visual artifacts.
It's a bit of a (very expensive) tradeoff. G-Sync assures only best quality at the cost of performance. No G-Sync assures best performance at the cost of quality. And the G-Sync illusion is only noticeable during moments when things would look very ugly (if they're displayed at all) anyhow. You can hope some NVidia driver update addresses your particular issue (good luck with that), and you can always turn G-Sync off (or buy into AMD and FreeSync) if G-Sync annoys you too much.
SLI in itself shouldn't be causing this (at least, not without also causing other problems noticeable enough to certainly determine it isn't working). It seems your GPUs just aren't manly enough for the task (when everyone starts throwing grenades and all that), so your options are to sacrifice a notch on visual quality or to buy a GPU upgrade.
Your x8/x8 GTX970 cards might not be quite enough to push a lot of grenades across your native 2560x1440 resolution. I run a x16/x16 GTX980 on the same monitor and I still need to sometimes turn settings down a bit (especially the 4K DSR type stuff). Image quality still appears awesome, lack of extremely fine detail doesn't really matter when so many pixels in your face are kept in motion.
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