Aslo if the fix above doesn't work i would guess that the regulator in the alternator may be fluctuating a bit from age. This doesn't mean its bad necessarily just aged.
You may be able to fix this by installing the same type of capacitor setup that people use when installing high end audio systems so that the amplifiers don't fluctuate the power on the heavy bass.
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1998 EB Expedition 4X4 200,000+, 1999 Windstar 200,000+ (Needs Head Gasket), 2008 Chev Colorado (Company Truck, Really sucks but it's free)
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