In my case Microwave Oven was operating on one of the frequencies. I changed the channel and it helped. Eventually I got 5GHz router (since my Airport Express receives both 2.4/5GHz)and set it to 5GHz. Now it is perfect. 5GHz is not very popular, not used in cellular phones or microwaves, and has advantage of penetrating walls poorly. It is big advantage for me, at least, since everything is within one room while disturbing sources at 5GHz are behind the walls (if any).
music drop out--new router
I use my Logitech Squeezebox to play music files on my computer via wireless router, which was recently replaced. I am getting frequent drop outs now, especially with high resolution files. Is this likely related to signal strength/reception from the new router? Is the best remedy a range extender/amplifier for the router?