Can you help with this problem?

Audiogoners can you help a friend who has the following problem.

Here's my set-up:
NAD 304 amplifier
NAD 514 CD player
Energy bookshelf speakers
Decent high-end cables... don't know the brand because I bought them
used from a stereo shop that was remodelling, but they are supposed to
be better than Monster.

I have a turntable, cassette player and mini-disc as well, but those
are the components involved in the problem.

Every so often, my left speaker will start to "sputter" and sort of
"fuzz out". I used to have the same problem with an entirely different
system (big, honkin' 1970s model Sony amp, Bose 901 speakers). I know
it's not the speaker cause I've swapped them over, and it doesn't seem
to be the connections at either the speaker or amp ends.
The only consistent thing I can tell you about it is that it often does
it with certain bass players... Charlie Haden will often cause it to
happen.
Is this something to do with the amount of power being put out to the
speakers, the frequency of the music or what?

Needless to say, it never happens when you want it to, so it's really
difficult to get anyone else to hear it.

TIA

Ben