Hallo, have you checked it personnally? Otherwise check it again. Disconnect breather and vent lines, blow it with pressurised air in opposite direction. Engine run on the ground is not sufficient.
We had this behaviour severeval times: fuel caps clogged, fuel shutoff valve halve closed etc.
Number one was in a cessna 182 with Conti IO550 conversion. Engine running perfect, but after halve traffic pattern severe power loss was coming up. Short story: fuel cock was in OFF position. The fuel was sucked backwards to the return line!!! After returning to BOTH position everything was fine. But this conclusion comes up after immediate landing and some brain storming. BTW an AD was arriving at the same time regarding desgn of fuel cock. The OFF position was eliminated then.
I hope everyone had a nice x-mas time and family time.
But we had also some airplane time…
First we used liquid nitrogen to cool the magnetos and the result on a ground run-up was perfect. The mentioned problems did not occur any more.
Today was the first flight with the new, still a little provisional, cooling system. Unfortunately today the weather allowed us only one hour’s flight, but we are sure we are on the right track.
I will keep you up-to-date!
@Willi66
At some point we also believed that the problem was caused by limited fuel supply, we connected an external fuel tank directly to the engine fuel pump and observed no change. Thus we could exclude that the problem was caused by the fuel supply within the fuel system of the plane. Fuel pressure was also measured with an external manometer and found OK, albeit at the lower range (20 PSI).
That’s a great bit of debugging!
However I still think there is something dramatically wrong going on (with the mags) because planes aren’t crashing all over the world. Countless Lyco engines of this type and/or with these mags fly both high up and low down. In particular, and notwithstanding that your type is overall very rare, the bulk of the Robin fleet is in France where it spends most of its time close to sea level 
and southern France gets pretty warm in the summer.
It looks like the mags are just faulty; probably some mistake made by a dodgy maintenance/overhaul shop on both of them. Maybe a bad batch of capacitors (condensers)? This sort of thing is possible. All kinds of weird things do happen; you would be amazed at some of the stories I have heard.
I would buy new ones from somewhere completely different (the USA, probably) together with a new ignition harness.
