Hi Undead,
I see you have UPS, thus you survive the power cut. Is there any possibility, however, that the problems you experience coincide with the power coming back on again...?
....My own experience with power cuts (I don't have UPS) is that I always get a dreadful sync speed after the power somes back on. I attribute that to the fact that everything electrical, from fridges to CH motors to DECT phones and the rest, are all springing to life at exactly the same moment, and the cumulative effect is - for a few moments at least - an untypically high level of electrical noise. Is there any chance that could be what's happening in your case, too?
Failing that, another comment would be that perhaps your UPS itself could be the source of the interference, although I see you've already said you can cut your own power with no problems. However, do any of your neighbours have UPS, and in which case I wonder, could one of their UPS's be the cuplrit?