I'm far from dismissing the Grundrisse. Read Workers' Liberty 3/16, entirely given over to discussing the Grundrisse and its relevance for today.
But the Grundrisse is rough notes - and, from what we know of Marx's work habits, a lot of them written late at night.
There are, I think, some brilliantly illuminating passages in it. There are also stretches of it, for example, given over to hypothetical arithmetical examples of economic processes, broken off short with remarks like "the devil take this wrong arithmetic!"
To make Marxism into a doctrine based on reverent exegesis of Marx's every scribbled comment is to descend from science into scholasticism.