GPU restart Mac Pro 3rd Gen. Early 2009 with GTX 1070, how to diagnose?
NVIDIA recommends that the system in which the card is installed should supply 500W: https://www.nvidia.com/en-us/geforce/products/10series/geforce-gtx-1070/

However, Apple warns that across all the PCIe expansion slots, not to pull more than 300W: https://support.apple.com/kb/sp506?locale=en_US

So, my guess is that whenever the GPU power consumption spikes, perhaps when you're playing games or videos, the Logic Board shuts off as a precaution.