I've had a very similar problem with mine before, which turned out to be a bad earth on the ignition amplifier. Might be worth checking.

Also, another common one is getting the plugs on the dizzy and crank sensor mixed up. They're both in the same place on the loom but one is grey and one is green, make sure the grey one goes to the dizzy and the green one goes to the plug on the end og the crank sensor.