I recently bought a B450M Mortar Max and Ryzen 5 3600X however when the PC boots it regularly doesn't start and the motherboard shows 3 error lights (CPU, BOOT and VGA). If i then cold boot the PC *may* boot up properly and load the OS. I can then run the system fine for several minutes (about 30 mins is the longest) before it will randomly cut off and the motherboard will show a CPU error light only.
I have done this with all peripherals removed other than keyboard and mouse and get the same issue.
Board: B450 Mortar Max
BIOS: E7B89AMS.270
VGA: Radeon RX 480
CPU: AMD Ryzen 5 3600X
MEM: 3x4GB, DDR4 Corsair Vengeance 2400MHz
PSU: Gigabyte GE-P720A-C2
To the best of my knowledge the above set up should be adequate and there are no compatibility issues.
Thanks for any assistance 
