Cat behaviour - biting, why?

[CodenameVillanelle]

So my cat is a 3.5 year old neutered Tom. He was neutered at 6 months. He has ALWAYS been a bitey and scratchy boy but I thought he would grow out of it.
He's lovely and affectionate and seemingly a very happy cat. He has a cat flap and a huge garden to roam in. But every now and again he just becomes vicious. It seems to be play but he doesn't hold back. His play bites break skin. He will run at my feet and attack them but worse is when he gets on my lap and I'm stroking him, he'll be purring then just get this look in his eye and bite my hand, hard.
We got him at 8 weeks which may have been too young? Why is he doing this? How can I make him stop? //😭