#1
To anybody who can help, please do!

I've been having problems with all three of my guitars recently, and I thought I'd fixed them all:

1) First strat copy, messed about with the jack socket and it works a treat now, as if it was new again.
2) Second strat copy, used some WD-40 in the pots and it sounds better than it ever did, all the noise gone. Had a slight action problem, but after consulting Total Guitar's gear guide I put the tiniest blob of superglue into the low e-string nut slot and its back to its good self.

Now my problem.

3) My Vintage, my pride and joy. Cleaned out the volume pots with WD-40 and completely resoldered the jack socket and it works a treat. However, when restringing my low e-string there was serious fret buzz from around the 15th fret back to the open string. Tried raising the saddles, still had fret buss from 12 down so i put it back to normal, after checking the intonation. Tried the glue-in-the-nut trick, didn't work, still have buzz from 7 down to open, and the string is as high in the nut as possible.

Please help, I haven't got a clue what to do next. If it helps, it's basically the SG copy manufactured by Vintage.
Gear:

Vintage VS6
Roland Cube 20X
Zoom G1X
#2
Try a truss rod adjustment, it sounds like you have too much bow in the neck. How worn are the frets? If the frets show signs of wear, they might need stoning.