I know about glxgears but it is very simplistic.. There are a few benchmarks out there like the one found here: http://unigine.com/
But you have to install the system to hdd, then drivers then the benchmark. But if this could be done by a live-usb it would be perfect!
Besides proper GPU operation is crucial to a system's stability. CPU, RAM, GPU and HDD all need to operate properly to classify a system as stable. Not to mention those who overclock their GPUs using overclocked BIOSes...
Thanks for considering it