Try to first see if something could possibly be scaring your kitty. A lot of agressive behavior *In people at least, I've noticed it somewhat in animals too* can be explained by something scaring them. Also, has your kitty been in a fight with the intruder? If he has, it might explain his behavior. Did you come in contact with the intruder cat? If so, he might be sniffing you and getting the odor of his tormentor...that can't help! Consulting with a Vet will rule out any deseases that might be causing his problem. (always a good idea if a stray wanders into your home and has contact with your cat) If the cause isnt physical, maybe a pet behaviorist can help. Most vets can refer you to one although they tend to be expensive. Hang in there! You have to esablish to your kitty that you aren't the aggressor or the intruder into his home.