Originally Posted by MouSe
According the info I got from a Full Size GM truck forum I'm on, the 6.5" speakers are bad. Apparently it'* very common for the OE speakers to rot out. This causes them to short on themselves thus causing the "intermittent" issue.
I believe the concept is that the surrounds crack or loosen away from the frame, thus causing them to run out of alignment and shorting the voice coils against the voice coil gap. There are a variety of replacement options if you'd like us to look some up for you.