If the ZDZ FAlkon ignition are anything like Vlach (they once were partners), then the ignition failure mode can be "no advance". This is the mode that the ignition uses when battery voltage gets low. The engine runs with both cylinders, but without advance and so limits the rpm. This way the pilot is warned that he should land as quick as possible.
Sometimes the ignition fails by reverting to this mode even when voltage is high enough.
Just to be sure: check your wiring, battery and switch under load and vibration.