"plugs read quite rich is it running lean" bit of a contradiction ,, maybe a vacuum leak at the manifold since only happening after the headers, the fact that you are making it richer suggests a vacuum leak, a really wet plug often will not come back to life unless the engine is running correctly to burn it off, and black soot will cause the plugs to short at the electrode, it might not be your problem but when mine was breaking down it was the coil, I removed the ballast fitted an electronic one off a commodore, small little black one, the engine will redline the tacho every time.