BoxOfSnoo: Sounds like you might have a bad motherboard... apparently the motherboard makers were a little upset with AMD, they had to rush their products to market.
clarry: It is far too early to jump into such a conclusion. Every part of the build is new, any one of them could be malfunctioning. Or it could just be a matter of tweaking something (dram timings again?) in bios, knowing the ryzen's memory controller is a little picky. Heck it could be a software problem. In fact I just swapped to another OS and the machine's been running for 15 hours without a reboot now. Maybe it helped. Time will tell.
I have a daily reboot with a Bluescreen for an unknown device. Driver Power State Failure, since I run the computer on Max Power in the System Energy Settings, rather than "balanced". It always just one time, so I guess some device is not correctly shut down on the previous day and causes a wake-up error. I have to pin down what it is, but I guess it is one of the network devices.
I have now finally got my RAM to run at 2400 MHz with rather aggressive timings (14-14-14-14-28). The trick was not to use the XMS profile of SPD, but that means the RAM is only running at 1,2 V rather then the specified 1,35 V. Well for now it seems stable (on Desktop, have to run some games now), but fact is the timings seem to be tricky beast as well. One less and Windows refuses to start, two less and the BIOS resets.
Well it is developing, what was to be expected as an early adopter. ;-)