my last go at throwing cr@p and seeing if something will stick… but hey I am here to learn 
you went 6 panels, then 12, and since after last bad load shedding, added 3 more (my incomplete hypothesis is though dependent on not having any daylight load shedding since then since the problem should have come up then).
any case, inspired by @JacoDeJongh 's post, with the extra panels, possible surge arrestor leaking to earth when MMPT/s MPPT/s sweep for power point (the sweep possibly explaining the intermittent appearing nature). But I do not know how the neutral will come into the mix.
There, I tried. 