@f1ee7a25 I'm wondering whether it might be bad piece of RAM, or possibly a mainboard issue.You could try installing Ubuntu on it, see whether their hardware diags report anything. Linux tends to be more transparent than macOS about what state the hardware is in. 

 @531f5a72 Oh that's a thought! Thank you, I'll have one more crack at installing MacOS from an external drive then look at that. 

 @f1ee7a25  Don't forget, be booted from the external to install. 

 @531f5a72 Roger!