Besides running your “stress test” for a longer period of time, I don’t know if there are many more tests.
If possible, you could try to swap parts of the system, e.g. the PSU, GPU, RAM etc. one by one from another system and check, if it’s still failing. Also, maybe switching the PCIe port could also help. A defect in the PCIe connection would usually just drop the GPU, but hardware defects are not always “well behaved”.