97 Yukon Elec Problems

My 1997 Yukon when I come to a stop at an intersection the voltage will bottom out. This also occurs when the air-conditioning is turned on and the truck is not in motion or moving at a slow speed. When I accelerate my belt squeaks horribly for a few seconds then the voltage returns to normal. I have replaced the alternator, belt and tensioner pulley, the problem remains. Note: the problem does on occur if the lights are off (no load on the power system).

Help :banghead: