could that be a grounding issue? sometimes it cuts completely out and sometimes only partially. i just replaced the speakers yesterday, which was surprisingly easy, so i know its not the speaker itself. it did it with the blown factory ones as well.

 

the wires can fail at the flex point where they come through from the body to the door.

 

Wire insulation is worn or cracked, probability in wiring that go's from the door to the car or stop breaking.