Why does GTA V combat stutter on a Gainward RTX 2060 Storm?

GTA V combat dropped to 48 FPS due to PCIe limits. I forced Gen 3 in BIOS, but the GPU failed. I used GamePP Hardware Sensor to set High Performance mode. 3DMark verified 15-16GB/s bandwidth, and RTSS showed stable frame rates with 62-68°C temps.