Oooo, I think it is Wheels on Fire by greyangel.
[spoilery description of the fic overall] It starts off with Buffy peeking around the mansion and Spike sensing her, they have revenge sex and Angelus gets really angry with them both, punishes Buffy (I think) but Spike and Buffy don't seem able to keep away from each other with Spike's demon wanting to claim Buffy. Soon after Spike starts being really aggressive often to Buffy though reaffirming the claim which comes across as him resenting the claim and the violence of it all upsetting Buffy. They test that it is a reaction to seeing Buffy specifically by getting her to step out of view then back again. Giles works out it was an enchantment Dru had placed on him that was strengthened in his dreams and the claim put him in flux, wanting to kill her but also claim her, wanting to be around her and nowhere near her etc. Giles/Willow broke it. | 
