Fluctuations of the loudness of speakers???

I've just discovered something that has me concerned. Sometimes while listening to music, either of my two speakers (that are probably not broken in yet)suddenly loses volume (not all the way but still very noticable)...it's like they lose juice suddenly. It's more noticable at lower volumes but happens at higher regardless. I don't know if to blame speakers, or cables, or if my power outlet has a problem with my 250 wpc amplifier. Sometimes a simultaneous "crackling" sound occurs. I have no idea if the problems are related but at keeps me up at night :-( Any ideas greatly appreciated...