i have the same problem with my 99. everything seems fine until i come off the gas and then everything just kinda drops, the fan speed, my subwoofer, and interior LEDS. i think it just had to do with the fact that the alternator doesnt put enough power to the battery to maintain the voltage. i am planning on upgrading my alternator pretty soon because the 400 watts i have running to my sub pretty much murders my electrical current when its turn up lol.
1997 Ford ranger. 2.3 ltr 4 cylinder 5 speed. 10" Kicker CVT, Alpine MRP-M450, Alpine CDA 9825. Flowmaster Delta 40 series. Smoked tails and corners. Black grille. Blue LED dash and interior backlighting. Front Fog Lights. Bad ass snowtires in the back.