Recurring short 75 RPM drop

Bing Carbs balanced?  Fuel Filters clean? 

Assuming its an ignition fault, when you do an ignition check on the ground do you get exactly the same 75rpm drop? If you do then it sounds like you are losing a complete ignition module. If you get a higher drop on the ground then the failure in the air could just be a single pair of cylinders which could mean a single coil or a single pick-up.

I would always start by checking the wiring and connectors. Then I would make resistance measurements through the various coils (trigger, charging and high voltage) whilst flexing the cables. Could you have an ignition switch fault?

I have had exactly the same problem several times with a 912 uls Tecnam and each time it was an ignition module. It would start as an occasional small drop in RPM for several seconds at any power settings and will get progressively worse till eventually it would drop out at idle and by quickly doing a magneto check, could identify the problem magneto. I had a spare module that I could swap over to confirm that was the problem.