This may sound like a stupid question, but here goes.

I have one cylinder not firing right. The header pipe doesn,t get very hot.

When I put a timing light on that wire, the light flashes just like the rest.

Could a bad plug do this?

Yes. Check it to see if it is fouled. If you are careful and can remove the wire boot from the plug, let the spark jump to the plug through the air about 1/2 inch. The spark will intensify and overcome a fouled plug. I could also be just a bad plug or plug/boot connection.
Check the inside of the distributo cap for carbon tracings which will cause a no-fire/ mis-fire situation.

That is where I would start. Good luck....