Rev Counter problem

After servicing my distributors (new points, condensers), my rev counter works only intermittently. Half of the time it shows 0, half of the time it works as it should. Engine runs fine.

Never seen this problem before. Can the problem be related to some cable not being properly re-connected at the distributor?

('74 GT4, dual distributors with points, single point setup)

 

Thank you, I will have a look at the coils!