Thanks MaJoc...

As far as I can make out, with my PC, it is an issue that can occur with certain AMD CPUs when IBS (apparently Instruction Based Sampling..) is enabled and the CPU goes into a certain power saving mode. AMD don't seem to have any plans for fixing it but it does not appear to cause any harm.

I think it would happen with Windows PCs too but I don't know how they handle it. Linux, as we know, can make some odd protests but, on this PC, I don't get them often, every couple of days at most.

As it stands, I'll just leave things alone.