In the late '90's I read a book where the main character would constantly reincarnate and have a life 'do over' until an inevitable heart attack that would always occur at the same date and time.

Each life would start a little later (he would re-inhabit his body at a slightly later stage). He meets a woman who is having the same experiences as him, and they have a relationship that spans lifetimes.

Each new iteration of their lives they look for each other, sometimes having to wait until the other person reincarnates before continuing on their research into their condition again.