Give firmware-git a shot. There are some fixes in it that are not in firmware yet.
If that doesn’t solve it: for “underclocking” you would have to set a kernel param to enable it. I had a good experience with lact. They also explain the necessary setup in their README. In my case the GPU was running with higher limits than the vendor specified. I simply clocked it back to these specs and it seemed to have worked fine so far. It’s a different GPU, though.
If that also doesn’t solve it: do you have Windows as dual boot option? Can you try the same games there? If that is also unstable, I would suspect hardware issues.