Lola,
This might be interesting to you. I have a stray cat as well. I took her in almost 8 years ago. She behaved exactly the same way as your cat is behaving. At first she was fine, but then she started literally attacking me. She'd bite, scratch, hiss... one time I woke up in the middle of the night and she was standing right on top of me, hissing at me. I would have to wear jeans at all times, because if I walked through the house with bare legs, she'd go right for my legs and bite real hard. It came to a point where I was a bit afraid of her. Then I started thinking about this, and decided that my cat was ruling the house and that I didn't feel comfortable anymore in my own place. From that moment on, each time she attacked me, bit or scratched me, I would yell at her, pick her up and literally "throw" her out of the house. I'd throw her food dish and her litter box out as well and leave her out for a couple of hours. Then I'd let her in and she'd be all calm. She is now almost 8 years old and she is the sweetest little thing, I love her to pieces and wouldn't know what to do without her. She is like my family. Two years ago, I was very ill, and this cat literally took care of me. She usually is out and about, but during the time I was bed ridden, she never left my bed, except to use the litter box and to eat. She would sleep on top of my head to "keep me warm", she had never done that before and has not done it since. She'd also constantly "wash" me. I have had so much love from that cat, you have no idea. It was just a matter of showing her that I didn't appreciate how she was treating me.
To this day, she still attacks pretty much all people who come in the house, but she doesn't attack me. I even have a friend who calls ahead when he's on his way over and I have to make sure that the cat is either outside or in the bedroom, because he's scared to death of her.
As I was telling my cat story to a lady onece, she told me... how strange, I went through the exact same thing with my stray cat. In the past years, I have heard several stories like that, from people who take in stray cats and they start attacking them. I am seriously wondering if it has something to do with the fact that they were a stray. Don't give up, but show the cat in no uncertain terms that his/her behavior is not appreciated. I bet it will get better soon.