Ignition Blues

[NATONEIL]

Ladies and Gentlemen
I have a miss fire on cylinder no 5. I have powerspark electronic ignition in a Lucas 45D6 distributor and after checking the plugs leads using a clip-on timing light I can see that I am getting an intermittent current on No 5 lead.
I then took off the plug cap and the engine runs on 5 cylinders with no misfire and again I can see that the spark is intermittent, so I am content that the problem is ignition on No 5.

Since then I have changed the Distributor cap and also swapped plug leads and the problem remains on No 5. I assume it is a problem with the electronics module in the distributor but is there anything else I should check whilst waiting for a new module and if a new module does not fix it what else could cause a problem like this.