[Rick Parker]

As Jerry hinted at, if the pulleys are underdrive your alternator may not be able to keep up with your electrical demands. However, check your battery for load capacity not just current voltage reading. Then, check alternator output (Which I suspect will be ok) and then voltage regulator ,many auto parts stores such as Auto Zone will do it for no charge. I suspect you have a fan(s) that may be drawing to much current or a short circut that is somehow temprature related and appears only when the fan circuit is activated. Since the amp needle goes positive upon startup (COOL) it would indicate alternator is functioning and regulator working. Keep us posted.