I assume you have tried all the positive parenting rules of time out, positive reinforcement etc etc? I have a three year old boy and a 5 year old girl and my son can show very aggressive behaviour, especially towards other children of his age and his sister, seemingly for no reason, and be the sweetest boy at other times. I wouldn't say I have cracked it yet but have seen great improvement simply by establishing much stricter routines into our lives, and also ensuring he sleeps longer hours. He seems to behave much better if a) he is rested and b) he knows what is going to happen when, especially if he can be given some input/cooperation tasks (help mum set the table, choose clothes for tomorrow etc). I know it must seem obvious to most, but it wasn't to me!

If you haven't tried it yet, it might be worth having a go.