1998 Ford Ranger Voltage Drops after driving for a short wh

I just replaced the alternator, because the previous one would drop voltage under high acceleration. The guage would show half way between the battery indicator and 0. But would rise again when the speed was dropped to like 60 Mph. The problem was the voltage regulator and the rotor. Were bad. I decided to just replace alternator.

with the new alternator, initially the voltage is right on target when the vehicle is first started. Indicated by the voltage guage splitting the picture of the battery diagonally. Then after a few minutes of driving the voltage indicator drops and crosses the bottom of the battery. And pretty much stays there.

When I shut the vehicle off and let it sit for a few minutes, the cycle is repeated.