Ive gotta say, Leads - Leads - Leads.
I had the same prob, doing 50-70km's, then flooring the thing, almost made it stall.
Also, when it was cold, Just taking off, the thing would have serious problems, so I always made sure not to pull out in front of any trucks. Get some decent leads, (not the Silcon Suppressed crap, or standard FoMoCo stuff). My Magnacores cost me $80 from American Auto spares, but check out the new forum on H.T. Leads. These were good for me, but It sounds like there are much better ones out there.
Finally, The EF doesnt have a Dizzy Cap, Only the earlier models, and the EL. The EF has a problem with Under Bonnet Temperatures, and the ignition. This is from what I can tell a common problem.
There is another forum somewhere here, with the same info - I think changing H.T. Leads or something. Might be worth while checking that out as well.