Help with Sonus Faber Extrema issue

I have a pair of Sonus Faber Extremas hooked up to a pair of Parasound JC1 monoblocks.  One of the speakers periodically will exhibit signicantly lower volume (down to almost no volume) in the middle of a listening session.  It seems to occur when during more dynamic passages and also when the amps have been on for a while.  If I turn everything off for a few minutes, the problem usually goes away.  I reversed the speaker cables at the speakers and the same speaker still exhibited the issue, so the issue is with the speaker.  What could cause this problem?  Could it be a bad connection somewhere in the speaker?  Could it be that crossover parts need to be changed due to age?