Reincarnation by Suzanne Weyn
Funny story, I took this when it was a stripped cover book at the store. Then bought it later at a school bookfair. So I was able to conveniently keep one copy at school to read at lunch, and then, when it turned out to be such a good read, I was able to read it at home as well.
This is the story of two, you might call them star-crossed lovers, people who keep meeting over time as they are reincarnated as different people in different times, and always something keeps them apart, or ends one of their lives right as things are about to start happening for them. It begins with the boy as a Neanderthal I’m guessing, and the girl as a Cro-Magnon. Then they are an Egyptian and a Nubian, then they come back in ancient Greece, and so on and so forth. Almost all the times we see them also seem to be either big historical times, or else very charged with some kind of racial tension. At one point the girl is even reincarnated as a guy, and the guy is reincarnated as a girl.
Fun story, and in the end, modern times, it seems as if they may actually get a chance to be together, although the story leaves off before we could possibly know for sure. But we are left with hope.