I just had something similar happen to me. I decided to try icing sugar from a bulk store. You buy however much you need. I got about 4 pounds. Made the buttercream.....grainy. Made some gumpaste......clumps like you wouldn't believe! AND I sifted before each of those!
Needless to say, the rest went into the garbage. Used my usual Roger's brand and, VOILA! no problems.
I guess there is something to say about sticking with the good stuff and the stuff you know.