MAH82
Flame instability can be cause by a number of issues. Much depends on the type of combustion system in use. You statement is that the instability occurs in pre-mix mode. Without knowing what manufacturer of gas turbine you are using and what combustion system you are using I can only say this: Premix mode is the first mode you will see trouble with flame instability because of the lean conditions that the gas turbine is operating. Flame instability can be caused by many reasons including gas fuel quality, (Lower Heat Value Fuel Gas, Modified Wobbe Index Number, condensed Hydrocarbon Carry-over etc) fuel nozzle design range (effective area, design pressure etc) fuel contamination, fuel nozzle damage, poor sealing of the combustion chamber, a change in weather conditions etc. Much more information would be needed to identify what is exactly going on with your gas turbine.
Kind Regards
Romefu12