Synthetics are poor at finding instability, they also likely have a dedicated code path in the driver to "boost" scores in benchmarks for better brand image.
Games will crash more easily. Downclock it but keep volts high, does it still crash? If not then it's hardware, if yes then it's software. Companies love to mess up VRAM and VRM cooling, causing issues with cards. NV drivers can hardly ever be called 100% stable, even their power saving features tend to cause issues in some situations where they fail to clock higher when necessary.