Assuming it is not when there is loadshedding
We once had that on a Geyserwise MAX, it turned out to be a dry joint on the PCB in the control box. That said, there are quite a lot of possible causes. This is also not a problem that you can leave as it could be indicative of a fire hazard type electrical problem. The first thing I'ddo is to ensure that all the screws in the terminal blocks are tight and that there are no loose wires.