such splotches come from the vraysky also, place vraylights at the windows in skylight mode... you then shoot a lot more samples into the room. and if the lightmaterial is not part of a multisubmaterial then check the directlight option it forces the lightmaterial to act like a vraylight and shoots samples (subdivs).
but I also have interiors where nothings helps... I think it's because of a material. really bad for GI are twosided materials on curtains or opacity materials. if you disable them from GI things get better.
