I'm not an expert, but it sounds like a hardware problem. I've experienced something similar when a ram stick failed. If you have multiple ram stucks, can you remove one and try booting? This helped me diagnose in the past. UEFI I don't think goes bad too often, but ram fails a lot and is easy to check.
Sorry that I can't help with your coreboot question.
I'm not an expert, but it sounds like a hardware problem. I've experienced something similar when a ram stick failed. If you have multiple ram stucks, can you remove one and try booting? This helped me diagnose in the past. UEFI I don't think goes bad too often, but ram fails a lot and is easy to check. Sorry that I can't help with your coreboot question.