Apparently there is a hard fault when accessing the last 500-700mb of the cards VRAM. Causing speed to drop and stutter the games. It may require Nvidia recall all of them to fix it ir reissue new cards.
Very few people actually had a game suffer from his because there are so few games that will use 3GBs+ of Vram. although I will say there have been a few issue on star citizen (known memory hog) where, 970s in particulare started to choke.
Other than that if your game only uses 2GB of vram and you never even touch that 3rd gb OF 4GB does it matter? (of course it does I'm playing devils advocate lol)
Yeah this sounds like something driver/firmware related i'd be very surprised if it's a physical issue. Will keep my eye on this because i was considering a 970.
I'd say the mid end sweet spot (960level performance) should be around 3GB with a 196/256bit bus MINIMUM. Cards above a 770 in power def. need 4GB. 256/384bit
I don't trust the majority of 970 owners enough to perform a firmware update. Especially considering to possibility of bricking it. If it does require a firmware update it will most likely require a recall.