Maybe she has a significant other and when things are well with him, she doesn't want you and when things are not well with him, she needs you. I had a friend like that, where when she was happy with her boyfriend, she forgot all about me. But then when he was cold and not giving her attention, she became very needy with me.

Whatever her reason may be, if the relationship is confusing for you, then I agree with you that you should not be close to her. You can downgrade that friendship to the level of casual acquaintance, and commit to maintaining that level so there is no up or down.