Hitting

Everett has a big problem with hitting Ashley. He never does it maliciously. It is as though it is an impulse he can't control, actually. He'll just be walking by her and all of a sudden he'll take a toy he is holding and bop her on the head with it. He never does it to get her reaction, because when she starts wailing, he's ignored her and already moved on to the next thing. He doesn't do it to get my reaction, because he doesn't pay any attention to me after he does it, either.

Sometimes he'll knock her over, or he'll sit on her, or he'll do other things, but mostly it is hitting her over the head.

When he does this I put him in time-out for quite a while, and sometimes I even yell at him if I'm frustrated (but I am always very stern with him).

I am trying to not give him too much attention for it, in case that is part of the reason he keeps doing it, but when I watch him, it never seems like he is even thinking about what he's doing. That is the thing I'm confused about handling. It seems like he just does it for no particular reason, even though he understands the consequences.

I've tried explaining it to him, I've tried punishing him, I've tried giving him lots of positive attention and encouragement, but nothing seems to make a difference. Two minutes later, he'll walk by her and BOP! Another hit over the head, and then he has immediately moved on to his next activity.

I am trying to train him to 1) think about what he is doing and to 2) choose not to do this particular thing, but it is not working. Does anyone have any suggestions? I am starting to feel at a loss! I know this is probably a stage, but short of whacking him over the head myself, how do I help him pay attention to what he is doing? And how do I get him to stop! I feel like the neighbors hear crying kids all day (Everett, having been put in time-out, and Ashley, having been hit and left alone while I'm putting him there).