I changed the fuel filter this afternoon as its age was questionable, and it appears to have largely solved the erratic idle! I would have expected the overall running condition to have deteriorated with a clogged fuel filter, but above idle it seemed fine.
So I say "largely solved" because it's not as smooth as it's been, but definitely a step in the right direction. Is there anything else fuel related that will effect idle? I did unplug the idle control valve (I believe that's what it is) on the back of the intake manifold to make sure it was in fact working, and the revs dropped and stumbled a bit more. Plug in and it reved, and then settled to a more even idle, so I know that's at least working.
1970 Alfa Romeo 1750 GT Veloce (105.44)