same issue here, I think it has to do with their new ad scheduling feature. At least form what I observed on my campaigns. I tried to use ad scheduling but conversions went down and the cost per conversion soared... still trying to recover lol Also I noticed a sudden the was a shift of focus from the kwds that were driving most traffic, needless to say the ones that started getting ore traffic were the ones with higher ppc bids. As well they started getting more 'assists'
I can't really track the date as I've constantly have been turning my Yahoo ads ON and OFF. It is almost like they're showing somewhere in a different world. I'll probably end up with shutting down over 95% of them (again).
Something is wrong with this PPC network, and until they finally let us pick REAL Yahoo Search only, my trust is limited.