[COLVERT]

You say it gets worse if you open the spark plug gap up.

Therefore it has to be spark related.

The voltage increases as the plug gap increases and gets less as the plug gap decreases. This might indicate faulty plug leads that start to break down at the higher voltage and will allow the spark to track through the casing to earth. It can also indicate a break down in the coil windings with an internal break down to earth.---