Hhmm I did underclock the RAM to 4800 MHz, since running it at the stock 6400 MHz would overheat the system (it's a mini PC) and cause artifacting. And, practically, I don't need higher frequencies, since I'm using the machine as an HTPC and for casual desktop use. In fact, from what I've read, high frequencies can introduce stability issues on these APUs, which is exactly what I'm trying to avoid.

But I'll play around with this and the timings, and check if there's a BIOS update that addresses this. Though I still think that AMD's drivers and firmware should be robust enough to support any RAM configuration (within reason), so it would be a problem for them to resolve regardless.

Thanks for the suggestion!

That’s almost certainly the problem. Play with the Linux CPU governor or (better) the package TDP (in the bios) instead.