2.1/2.0 Speaker Systems (READ ONLY ARCHIVE)

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I have a set of Z313 Speakers and they were working great for a time.  But now for some reason the left side speaker stops producing sound after a half hour or so.  I've checked the balance on my sound card and everything is right down the middle.  I've learned that if I unplug the cable from the sound output then plug it back in the left side speaker will kick in again for another half hour or so before it goes quiet.  I'm very happy with the speakers when they're both working, what could be causing this strange behavior?

Not that I think it will matter, but I'm on a MacPro running 10.5.8.

Thanks for your help.