Car Stereos: Speakers keep shutting off at a certain volume point, factory speakers, aftermarket radio

Question
My father and I have had a 93 CK 1500 Chevy for almost 8 or 9 years. We put in an aftermarket radio almost immediately after we bought it. We have never had any issues with the system until lately. i was driving down the road when the speakers just simply shut off, however the subs continued to work.
I replaced the amp, speakers, and the stereo. However the problem still occurs and i can not figure out why. Any suggestions?

Answer
JJ, I would think if the subs are still working, the problem lies with the factory speakers or the factory speaker wiring.  Many times the wires going into door jams wears a bare spot into the wire, and causes a short.  I would start there.  If you can't locate a short anywhere, I would suggest running new speaker wires to all factory locations.  HOpe this helps, Scott