Sounds like a wiring issue to me
Have you got a volt meter? If so turn the ignition on and check you have voltage at the coil. It sounds like the coil is not getting a power feed with the ignition on but is getting one when the key is in the start position. You have either wired it wrong or the ignition switch is playing up.
there should be two black (w/white tracer) wires going to the coil + one from the dizzy and one from the loom.
have a look in the stickies at the top of the electrical section (wiring a vit 8v into 410 and there is a useful wiring diagram
![Mr. Green :mrgreen:](./images/smilies/icon_mrgreen.gif)
) they could help you track your problem.