Ian

Dave already responded and I agree with this direction:  Too lean

The last time I diagnosed that (by remote), it was an electrical issue.

1.    What size idle jet?...probably a 42 from the factory

2.    Is it loose?

3.    Have you put a safety pin into the orifice, to clean it out?

4.    Any junk in the carb bowl?

5.    Stock needle?

6.    Although the main jet should not have much affect at low RPMs,
what size is it?

7.    I read that you already checked for intake manifold leaks and
checked the plugs...what color where they?

8.    Timing?

Got to go to a meeting, if I think of more, I'll repost.

Jerry