Flightless Fruit Flies....learning how to fly again?

I am having some severe issues with Flightless fruit flies becoming gifted with the ability of flight. I can't understand how in the world this would happen. The species of fly is Melongaster. When I get the cultures they are all flightless and doing great. Then all of the sudden I notice that when all of the newbie flies hatch out some have flying capability. I immediatly throw those away and hope that all of them are not like this. Now I have talked with my supplier and they guarantee that its not their stock. So how are they becoming fliers again? Too me it would make sense that it would be the stock but some have said that temperature can turn them into fliers again. I just want to get a genral consensus and see what everyone elses experience is. I know for a fact that they are not wild species because I live in CO and its a little too chilly for them right now. Also I keep them at a temperature range of 65-75 degrees. I also tried placing half of them into a dark area and others out in the open just to see if that would make a difference...but guess what? It hasn't. Any help on this would be very helpful. Thanks.