Dave, thinking about it some more whilst working I think I'd look at your coil first. It really is presenting with a failing coil symptom so you might be getting a spark and it might look fine but it could be weak and this presents with fuelling type problems but its electrical. You can test the coil with a multi meter, low tension side, ie little terminals pos to pos and neg to neg multi meter leads on a low ohm setting and you are looking for a steady reading below 5 ohms HT side pos to pos terminal and neg to king lead hole on a 20,000 ohm setting and if memory serves me correctly you are looking for a steady 300-500 ohms reading. Most return about 380-420 ohms.