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...
Sounds to me like it is something other than your speakers. Check your amp/preamp.
Most problems with electronics involve connections. I would clean all connections (the Caig A/V Survival Kit does the job well). Then treat all connections with Mapleshade SilClear or similar. That may solve your problem. If not, you may have a bad interconnect. Try changing them out one at a time to eliminate the bad cable. Beyond that, you may have a bad tube, (if that applies) or something else internal in amp/preamp.
Recheck your cables...(interconnects and speaker) - make sure they are clean and tight.
Cables are all brand new but the tube does apply since I have Musical Fidelity Nu Vista amplifier that supposedly contains some tubes. I wouldn't even know how to go about getting let alone replacing one...
most likely the amp
Do you guys think a bad tube would probably be responsible for the volume fluctuations rather than an occasional "crackling" sound? With rock music hard to notice, but put on some ambient music, and it's more noticable...thanks everybody for their (even future) inputs and ideas.
Most likely a bad tube but could be a bad solder joint on the output stage.
Re E-Mail, you can just reverse the speakers by reversing cable from left to right speaker and right to left. If the problem is still anchored to the same speaker then its not a bad tube. BTW, I've never experienced fluctuating volume from a bad tube, but that doesn't mean the problem is not in your amp or in a source. If you can, change sources and see if the problem goes away.

Also, are you detecting a shift in the stereo image toward the same speaker when the volume fluctuates, or does it stay centered. If it stays centered then the problem is not likely to be from speakers or, if you have two or more tubes, one for each channel, is it not likely to sourced to a tube. Its to coincidental to have two tubes failing at exctly the same time.

Hope that helps a bit.
What you're describing are the classic symptoms of a bad speaker relay in a power amp. That is:
- Can appear in either or both channels
- Changes if the amplifier is cycled on and off
- Much more pronounced at low volume levels
- Cranking up the volume for a few seconds fixes it at lower volumes for awhile
- tapping or pounding on the unit can cause or alleviate the problem

Of course if you don't have a solid-state amp with a speaker relay . . . then that's not the problem, in which case I'd second others' suggestion of bad cable connections, or loose solder connections inside a component.
It seems like I'm going to have to borrow an amplifier for a day from somebody (nobody I know has any) and see if the problem continues...then I can apply the process of elimination. What a drag...I'll be sure to post results when I figure it out or even as I get closer. Thanks everybody for their suggestions...
Another observation, problem is more common when the system is cold than warmed up...
To change a tube, open the case, just pull out the tube when cold,,,pull straight up - don't twist it... and replace with the same tube type and number. You might have some dirt on the pins of the old tube, so I would remove and replace the same tube to see if the problem goes away.
Well, it's not my speakers...just borrowed generic receiver and none of the problems described above occured. The music sounds like from behind a curtain though, all the details are lost. Made me realize that apart from the possible problem with my amp (tube?)it blows the generic Best Buy stuff out of water :-)