I've got an old guitar (Yamaha SG200, tunomatic bridge, Les Paul/ SG style)

The guitar isn't in great shape, and soon it will be getting a new nut, and the frets leveled and crowned... I've also blu tacked the s##t out of the bridge, to stop that retaining wire buzzing (I think it's the wire...)
But that's all understandable (it's 30 years old!), however, one string (G, un-wound) continues to infuriate me- it's got virtually no sustain past the 10th fret, and when it's ringing on any note, it sounds `off` there's no other way to describe it!
The D string and B string, either side sound pretty smooth, the G seems to change not slightly, back and forth (very slightly) while it rings, and just sounds muddy etc, this comes through the amp, and really affects the tone, and playability of the instrument.

I'm thinking it might be to do with the saddles...

(If that's the case, will standard tunomatic saddles go one, my bridge post spacing is 76mm, higher than that of a Gibson style)

Lubricate your nut, check your saddle for burrs, new strings, intonate properly, come back if the problem's not fixed.
Intonation is dead on
Had this problem with old strings
Nut has graphite in, but nothing else- should I put some Vaseline in there?
Will have look at saddles soon!