Trying to find an old book

I was hoping someone could help me. I am trying to find an old book
or novella about a woman who is driving across country (from New York
to Los Angeles) with her two kids to meet her fiancee who lives in LA.
While driving, she begins to believe that someone is stalking her and
is trying to kill her. but since she is travelling between states,
the police can't really help her. At first she thinks it is her
ex-husband, but realizes that it is not. She begins to wonder if it
is her fiancee. I don't know how the book ends. it might be an old
Hitchcock presents book, but I could be wrong.