1991 Oldsmobile Cutlass charging

91 olds cutlass, some of the time while driving car, voltage will drop 2 bars from red, lights will dim and other electrical units will slow, I have no aftermarket products installed, when I push on the brake pedal the volts go to normal and all other electrical devices work normal, as soon as I release brakes the volts go back down. Also when driving vehicle, volts will go low than jump to normal than back to low and back to normal, other times when I slightly press on gas to maintain speed voltage will drop, when I let off the gas volts will go back to normal. Help, thanx in advance.