hmmmm?

it could be your engine loom, this was what it turned out to be on my car

my loom was baisically old and fooked, it had shorts all over it and it keeped blowing act sensors like there was no tommorow!

ok so have you checked all of the following?
crank sensor, dizzy cap, dizzy phase sensor, map sensor, plugs, leads, all vacuum pipes off the plenum?

when its on idle and you pull the plug leads off one at a time do they all make a difference? ie, is she firing on all four cylinders?

when my loom was shaged i had the exact same problems as you! missfire at around 4k but it would clear sometimes when you boot it!
was overfueling like a good un!