Sounds like you are experiencing a right conundrum!
Working on the assumption that your Genoise is the traditional Genoise as described in Larousse (ie addition of melted butter), can I suggest the following method changes:
1. In your Viking bowl, using a hand-held mixer, beat your eggs and sugar over a pan of simmering water until the mixgure has quadrupled and the sugar dissolved. Bring back to your machine.
2. Using the whisk attachment, keep beating the egg mixture, and add the melted butter at this point, and beat on medium until the mixture is cooled and well thickend (it will almost resemble whipped cream). This change in method will stabilise the air in the eggs and allow the mixture to accept the flour without the loss of excessive volume.
3. Don't mix the flour in by hand. Add it in 3 parts using the whisk attachment on the Viking, using short bursts to incorporate the flour. Then, once 90% is incorporated, remove from the machine and give a final 20 strokes to incorporate all flour (including from the bottom of the bowl) by hand. Pour immediately into your pans to bake.
The traditional Larousse chocolate genoise recipe is as follows:
Melt 5-1/2 oz butter.
In a heatproof bowl, mix 3 oz superfine sugar with equal qty cocoa and 4 eggs. Place over simmering water and whisk until greatly increased and thick. Remove from heat and continue to whisk until cool.
Fold in 4-1/2 oz AP flour and butter.
Pour into prepared pans and bake at 350 deg for approx 25 mins.
Make sure you check back in with your next results! Good luck!