I do not know what failed on the first MB, sending it back for repair today.
Yes PSU working well and so is the memory.
I have two MSI RX3800 cards set up in cross fire and have had problems with them after I updated the drivers.
The second MB failed after I refitted the second MSI card and was trying to get the screens the right way round, ie monitor and HD TV. I was starting and stoping the computer a number of times then iw would just not start one time. All the cooling fans run including the CPU one.
Bob