You could try:-
Charms of the Clarion by Eurydice at Elysian Fields. The summary is:-
When the Council approaches Buffy with an offer she can't refuse, she finds herself in another world, with Spike as her necessary partner. Set early Season 5.
West of the Moon, East of the Sun by KnifeEdge also at Elysian Fields. The summary is:-
I keep having the strangest dream. Every night it's the same thing: a dark room, a big bed, and a silent vampire that I can feel, but not see. I'd be wigged, except it's just a dream, right? It'll be a cold day in hell before I ever willingly sleep with a vampire again... Adult Language, Buffy/Other (Temp), Character Death, Spike/Other (Temp), some explicit material
Also at EF is Interpretations by Suzee. The summary is:-
Spike is absolutely certain that after tying Buffy up and ranting at her and Drusilla, the Slayer's going to want nothing to do with him. But when a witch curses Buffy and then she comes down with a mysterious, possibly deadly ailment, he might be the only one who can help
There's also The Deluge by evenstar at EF. The summary is:-
It never rains in Sunnydale, until a powerful force prepares to rise from the depths of a second, watery Hell mouth....oh yeah, n' Spike's just come back from Africa.....post-Season 6
At EF there's also The Better To Bite You With by Joyful Dayz. The summary is:-
Buffy has discovered that The Hellgod Glorificus captured and tortured Spike in an attempt to make him tell her where the key was.
At Bloodshedverse there's A Different Light by dreamweaver. The summary is:-
During the fight over the Gem of Amara, Buffy and Spike get pulled into another dimension.
Also at Bloodshedverse there's In League With Serpents and the sequel Feathers & Forked Tonguesț. The summaries are:-
In League With Serpents - When Spike wins something unexpected over a game of cards, he ends up with a most unlikely crypt-mate and a mission to help a newly resurrected Buffy. With an empathic serpent setting up shop in his head, a scheming warlock on his tail, and a bothersome change in his relationship with the Scoobies, Spike finds himself in the position to save the slayer he loves, the world, and maybe himself in the offing.
Feathers & Forked Tongues - Spike and Buffy make a strange discovery during what should have been a routine search. While the Scoobies try to identify their find, Spike tries to figure out where he stands with the slayer. If only strange dreams, a matchmaking coatl, a teenager's idea of "help," and the vampire's own insecurities would stop getting in the way.
At Bloodshedverse you could also try Spike's Will Be Done & the sequels True Colors, Ring of Fire, Fumbling Towards Ecstasy and Angels and Demons. The summary for SWBD is:-
An alternative view as to what might have happened in Entropy if Buffy had given all Willow's spellcasting paraphernalia to Spike for safe-keeping instead of throwing it out. This time round, Spike has his heart set on the idea of casting the "Will Be Done" spell before he even reaches The Magic Box. What will the vampire wish for? And knowing the luck our favourite vampire seems to have, should we be surprised when things don't go quite as he planned? (Note, most of the times when Spike "shows his smarts" happen in some of the sequels).
At Nocturnal Light there's Masks and Mirrors by Pfeifferpack. The summary is:-
There's a glitch using those Ghorah eggs as a surprising result develops from Dawns attempt to bring back her mother. Those developments both thrill and horrify Spike as the scoobies and Buffy discover the man behind the mask. Can a mothers love heal and bind or should the past be left in the grave? Setting: Season 5 mid episode "Forever" going AU from there. Liberal changes made to the ritual Dawn uses for the resurrection spell.
At spuffystuff . org there's The Bodyguard & the sequel I Would Die For Youț by slaymesoftly. The summary for Bodyguard is:-
A Dracula fic that plays fast and loose with timelines and canon, setting his visit earlier than Season V. Angel-bashing takes place, as does a bit of Riley-bashing and some Spuffy UST.
I believe you can find this one here but if not you can at Bloodshedverse, it's The Ghost in You by BuffyMeetsSpike. The summary is:-
Goes AU during Bargaining. Willow's spell goes awry, leaving Buffy incorporeal, and visible only to Spike. With the Scoobies convinced that Spike has lost it, how can he help Buffy get back where she belongs?
Some of these are more along the lines of Spike using his brain rather than having forgotten knowledge, although The Better to Bite you With does have him knowing stuff about slayers that Giles hadn't known (nor had the council), but they are all really good stories. My special favourite is In League with Serpents.
Hope you enjoy them!!

Sarah