Gas engine shutdown.

I have wilson gas power station, Power Wizard 2.0 and woodward 2301D. Engine in island mode max loading power 43%. The engine stop: alarm Low voltege. I connected EST monitoring: 1) eng speed. 2) voltage. 3) generating power. I analised this grafic and saw: speed go down first from 1500 to 1000 rpm and after speed rise to 1500. because of low frequency avr stop working and genset sh.down: low voltage. I make video where measured voltage dc from woodward actuator output and motion of actuator gas valve at the same time. Engine load constant 43%. I saw before sh.down: actuator do not try close gas valve when failure revolution appear. Woodward 2301D rise output to actuator at that time. I think woodward and actuator is ok. But what is the problem may be? I have ignition system: altronic disn 800 maybe the problem is there. But how can i check it?