Setting the irradiance cache to 100% means don't use it. It means you want 100% accurate lighting, not interpolated at all via IC. Unfortunately, without interpolation (which smooths the variations) you get visible grain from the local per-pixel inaccuracy when you don't use enough samples to converge on the correct (and therefore consistent) value.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)