Nice find. Just ran the test, and same result. Last gig of ram drops DRAM-Bandwidth performance from 153 GB/s to 13 GB/s and L2-Cache-Bandwidth from 473 GB/s to 13 GB/s. However, i then proceeded to turn back and forth off and on the display composition (aero effects, etc) and the 153 and 473 results would periodically appear for some of the 3-4gb runs. Windows sort of freezes during the 13 GB/s results.
I don't think it's a 970 exclusive problem then. Perhaps their flawless 980 result and therefore inferring the 970 is the source of the fault is being mistaken for something else in the system causing it (assuming they were both ran on different pc's). Similar to the rf2 identical graphics card performance discrepancy problem that was being masked by the effect of PCI-e gen in rf2.
Read my previous post, this rec.exe shouldn't be executed with the video plugged on the VGA. It demands a headless testing using your onboard GPU. I got totally different figures after that, no more capping on 980 GTX after +3.5 VRAM usage. When you're gaming, Windows releases all VRAM for the game executable (at least you're running the game in windowed mode) so there's no bottleneck. Problem is, some guys are detecting this issue in 970 GTX even in headless mode (it doesn't happen with 980). There's a lot of misunderstanding in this regard, found tons of BS in internet forums propelled mostly by ATI fanboys rejoicing by a alleged Nvidia failure and people running the test in non-ideal conditions.
I cannot assure you'll get the same results, but if you want to check it out, you need to go in your BIOS and enable the onboard graphics, unplug the monitor from your 980 and plug it in the mainboard video socket. Thus, Windows will alocate VRAM from you onboard GPU and let your 980's VRAM free, then you can run rec.exe. I'm very confident your slowing down issue when overloaded VRAM will disappear. PS I'm assuming your mobo have onboard video chip. If you have not, the only way to check it is overloading VRAM in some game (rFactor 2 doesn't appear to go beyond 3 Gb, you'll probably will need another high-end graphics game all maxed) and evaluate the FPS degradation.
