[timattrn]

BIOS is the latest: I flashed a few days ago. When it boots, the system is very stable. However, sometimes the boot fails. Fans start, but there is no display.
I was using an RX570, now an RX6600. Powersupply is 1000W Corsair, power is supplied via a UPS. The case has good cooling and there is no chance of overheating.

Also, I have four DIMMs installed, identical RAM with RGB support but only banks one and three have RGB lighting. I don't think this is normal, however, it has been like this ever since I added the two new DIMMs (months ago). There is no hint of memory failure when a boot is successful.

What diagnostics are there? I don't have any speaker attached but perhaps this is what I should do.

This is the first PC I have built in years.

 

[Blameless]

It's a memory training issue. Likely could be resolved with adjustments to certain memory settings (voltage, termination resistance, output impedance, etc), or adjustments to SoC, DDG, or DDP voltage.

 

[Rykoshet]

I had a very similar issue with a x570m prime and it turned out to be my CPU. All signs pointed to GPU or motherboard or memory, but ultimately the 3800x had died.

It was a case of needing twice as many components so you can individually test each one

 

[timattrn]

thanks. I reset the bios back to defaults. Boot seems reliable without XMP on. I turned XMP on again, and I will watch over the few days. Two of my my four Corsair Vengeance RGB modules do not have RGB working. In my first post I linked that to the slot the DIMM was in, but by rearranging the memory, I can see that it is not the slot, but the module itself. I don't know what to make of this. When the machine boots, the memory performs well, it will runs days without issue include under quite a lot of memory load.