When a belt starts to slip after working for a long time, i reckon it should be replaced. Also, belts need to grip - screeching is them slipping, so putting on a lubricant like WD40 will just make the slipping problem worse. I wouldnt expect it to be bearing, as that noise will be consistent not coming and going as you're seeing. Overtighening usually just makes bearings wear faster - AFAIK it cant cause screeching.
Have you checked the PS fluid level? (not that I would expect that to make the noise you described, but i dont have any good ideas).