I built my system a while ago, custom water cooled AM4 platform. It was working perfectly, I changed out the CPU waterblock and swapped to vertical mount GPU then the problem began, from 'cold' it will throw POST errors and hang, pressing restart over and over it will hang further and further into the POST until it does eventually boot. Once its booted up you can shut down and restart without issue. Next day after its been off for hours I have to go through the same process to get it to boot.
I have just reseated the CPU and its waterblock but this has not helped, any ideas? I have never come across anything like it! Have already reseated the RAM and tried with the PCIe extension unplugged, behaves the same way.