Better late than never, I think it could be 'Becoming' by Mia Vaan.
Summary: Peggy Sue. When Buffy jumped off the tower in self-sacrifice to save her sister and the world, she didn't expect to find herself, moments later, in her seventeen year old body and locked in battle with Angelus.
Buffy sends Angelus in Hell quicker thanks to the time-travel knowledge and Spike gets cursed instead. She has to knock Spike out in order to take him with her to L.A. using his car. She chains him to the sink in the bathroom because he's suicidal.
This story is not available online anymore but I have it. Send me a PM if you're interested.
