Sounds a strange issue & one to which I don't know the answer.

Since the first shots impact away rom the point of aim in a random fashion does this suggest it might not be a power consistency issue? Wouldn't a small varriation in power just produce a degree of vertical stringing? A large degree of varriation might not stabilize the pellet & give varrying points of impact but wouldnt that be discernible in use. A small varriation in power might give rise go vertical stringing but at 6 yards I think it would be fractions of an inch. As I say, don't know thw answer but the results of ghe first few shots of a chronograph might be interesting. It does sound like something is settling down after a few shots...