First, a vet checkup is always a good idea when you see sudden personality changes. Cats hide pain and illness really well, and often it comes out as what we interpret as aggression or attacking behavior.
Look for patterns (I think I may change my handle to "Looks For Patterns"
). Is it a room, a person, a time (feeding and litterbox times are always suspects), is it after he's been looking out a window or outside? Is it when he's being patted or touched in a certain place or way? Is it something you're wearing (Wellies are one of Assumpta's triggers to turn into death-kitty of doom). Is it helped or made worse by playing?
Take special note of his body language and the way he holds his tail, ears, whiskers, and trunk, because those can give you clues to his motivation. Staring at an aggressive cat is uaually a good way to escalate aggression, so if you can, turn to the side, make no eye contact, and yawn openly to indicate that you're no threat (just a personal safety note there, because most of us humans will quite naturally stare at a hissing, yowling cat, which gets interpreted as a threat by the poor cat, who just gets more upset).
But I'd ring up the vet first, to make sure that he's not in any physical pain, then start looking for patterns and common threads in these episodes so you can avoid the triggers.