Fantasy/SF: centaurs and angels

I say a thread in FantasyFans about centaurs, and I remember reading a book that had centaurs in it probably about 20 years ago, but I can't remember the title or author (clearly, or I wouldn't post here).

What I do remember:

1. A female protagonist visiting some contraption in the sky.
   
2. I think she did talk to someone in said contraption, but it didn't go so well. In any case, on the elevator ride back, the floor opens up underneath her. She falls out.
   
3. She calculates how long it should take her to hit the ground, and it is a considerable amount of time, say half an hour.
   
4. Sometime during the fall, a winged man approaches and offers to save her if she'll have sex with him in flight. She refuses, but they come to some sort of agreement and he rescues her.
   
5. Later, on the surface of the planet, she meets centaurs. I don't remember the specifics, just that the first time she sees a centaur, he warns her to be careful, because he's worried she might fall over, having only two legs. So there probably weren't too many humans on the surface of this planet, or at least not in the region where she came down.
   
6. This might be from another book: there's a scene in my head, which would have been before the protagonist goes to the planet/sky contraption. The protagonist is walking through a hallway in some spaceship or space station. Her hand starts to shake. This might have been due to some medical condition? She sees this as weakness, takes out her knife and holds it to her hand. She threatens the hand and tells it to stop shaking, or she will cut it off. The shaking stops.
   

Does this sound familiar to anyone?

I'm pretty sure this is the Gaean Trilogy by John Varley which starts with Titan.

I think you may be right, @Mart1n! I'm not a hundred percent sure, but I've been reading some reviews and the wikipedia summary, and some things start ringing a bell. I believe there was a Gaea, and the ending with the protagonists becoming a wizard also sounds familiar. Thanks!