> 
> > You can easily measure if the amp is at fault by measuring the AC
> > voltage at the amplifier's output.  85->95 dB is a ten-times change in
> > output power, or a 3X change in output voltage.  There's no way your
> > amp is changing in gain by 10dB.  And your signal source shouldn't
> > either, if it's at all good (test by watching the voltage at the input
> > of the amplifier)
> 
> We've watched the signal source, and frankly, I doubt an Agilent
> signal generator is going to have much drift in frequency/amplitude.

If Agilent is still using Bill Hewlett's light bulb it shouldn't! ( if
you don't know the story, see
http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/index.html)

Have you measured the output of the amplifier when it is misbehaving? 
I suggest using an AC voltmeter on the amplifier output to set the
level, rather than relying on a volume pot.  Modern Best-Buy-quality
hifi tends to have an S-taper "goes to 11" volume control that makes
it hard to adjust at low levels.

Now my theory is something mechanical in your loudspeaker is
intermittently rubbing.

Regards,
Mark
markrages@gmail
-- 
"We act as though comfort and luxury were the chief requirements of
life, when all that we need to make us happy is something to be
enthusiastic about."

- Einstein
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