Spontaneous inequilibrium in a gas in an isolated system? Can an isolated system(of several phases) in equilibrium by itself move out of equilibrium?
if a system contains for example an inflammable gas dispersed with another combustible gas such that the molecular speed are insufficient to cause the reaction. but as the molecular speeds keep fluctuating, at some point, it may cross the energy barrier and may start the reaction which will eventually spread evenly but till then the temperature there would be high and thus the system in inequilibrium.
isn't this also an example that although the free energy is minimum and entropy maximum, some useful work would've been done if the temperature gradient had been utilised?
The short answer to the question "Can an isolated system(of several phases) in equilibrium by itself move out of equilibrium?" is NO.  Once the system is in equilibrium it will remain there.

If it is possible, as is asked in the second part of the question, to have some reaction be triggered at a higher temperature reached by fluctuation, then as has been commented, the answer is that the system was never in equilibrium.

We should never forget that there is no "absolute" starting temperature for a reaction.  If the reaction is physically possible, it will take place, but the rate of that reaction can be horribly slow.