wow...same exact thing is happening with me, same EXACT thing, new chain, new cassette and slipping on the chainrings when i crank hard out of starts usually. what i have diagnosed is that its the cassette not being tightened fully. wheneven it happens to me, my cassette seems to be loose. i tighten it, and the problem goes away, but when it reloosens [i never tighten the cassette down hard cause i dont have a chainwhip and its a pain to get off if its tight], the problem starts again. just now before i read this post i tightened the cassete really tight so itwont loosen again. we will see if this got the problem to stop on my ride tommorow.