sound the same problems are was having with mine very similar setup also with a L8 with pectal super8 board and RS500 inlet, etc

I bought new 909 leads, cap, arm & 017C plugs, replaced the Ignition Amp also with a brand new item and cleaned off the old silicone compound off the aluminium backing plate and applied new compound this totally solved the problem.

Before hand sometimes it would drive threw it, but it got worse at night, the misfire doesn't do ya engine any favours either so be careful

I also fitted new earth from side of the head to battery and checked , cleaned & replaced all connection on the alternator, by doing this i picked up over 0.65v at the battery when light where on and engine ticking over.

I for 1 would replace the leads cause they make look ok but they could well be arching out under load.

also without fail check all sensor gaps & condition as well as loom condition and voltage at ECU