A couple of days ago I turned on the kitchen upstairs and it worked for a second and then very low water pessure. Had a similar problem in the upstaris bath sink though it was the valve now I don't know with this new development.

Have you checked the aerator at the end of the kitchen spout? Remove this (clockwise) and clean.. replace.. did this fix? Check pressure without aerator.. If good may need to replace the aerator. Let us know how it works out.