Ok quick summary....car was running albeit with a few idle issues, changed gearbox (due to other problems not because I thought it would fix the idle


The problem appears to be spark related.
The car has new plugs, new rotor arm, new distributor cap.
If I pull one of the plug leads and using an old spark plug check for a spark I seem to be getting an odd, fairly week spark, just once when the engine first turns over but then no further sparks. But this is a fairly hit or miss occurrence with there often being no sparks generated at all.
I am struggling to work out why it would suddenly start doing this when it was previously starting no problem at all.
So most likely causes?