When it comes to troubleshooting a PC, there is no way to find out exactly which piece of component(s) is causing the problem.
You will really have to actually test out each parts to find out what's actually causing it.
So to ask if the cards froze , but not onboard, what should you do? No one can give a definitive answer for it.
All I can say is get a 3rd graphic card, change psu, reinstall OS, try one ram at a time, change ram, change board, etc.
Basically there are others out there with similar problem like your having, and each method of actually solving the problem will be different from one to another.
Let's say another user has the exact same setup and problem you're having and his problem is solved by changing the Power Supply. Does that mean your problem will also be solved to by changing the power supply? Maybe yes, maybe no.
Try this, go into bios and change the PCIe x16 mode from Gen 3 to Gen 2 and see if that works.