One of my fosters was a nasty biter when I got her. I would be petting her and she would seem to be enjoying it, then she would just turn out of the blue and bite hard, not just a nip. She had a hard life before I got her, including possible abuse, and she was emaciated and has scars, though she is not feral.
My teenage son and I started doing what we call the move in, move out approach. When she comes up to us for petting we move in and pet her 3 or 4 times, and then move out while she is still enjoying it. The chest also seemed to be a trigger area for her, as well as the base of her tail, which I thought most cats liked. It has been 5 months now though and in the last two she has seemed like a different cat. We can pet her quite a bit now on her face and all the way down her back, and she sits in our laps.
The best thing seems to have been when we brought home a kitten in November. She adopted him, cleans him, plays with him (she had babies that died when she came into our program) and it seems to have made her a gentler cat all around.
I second the idea of getting her mouth checked out and a general health check as well, but if there aren't any physical issues she may just need time, and in her case it may also have to do with being declawed.
Muffin, Zoey, and Cheddar Biscuit