Thierry de Coulon composed on 2024-12-04 18:28 (UTC+0100):
As I said in my answer to Nick, memtest stops at 27% when testint the L1 cache. I guess that means byebye processor...
Overheat somewhere is another possibility: power supply component, motherboard component, CPU or GPU.
I fixed a 3 month's of no-POST motherboard yesterday by changing one of its two 25.000MHz crystals, transplanted from another dead motherboard with identical CPU socket and support. The old somehow, unknown when, acquired a small case dent, so I took a gamble, and it worked.