My 1987 D-150 (new to me) was having charging issues. Replaced a bad alternator and then a bad voltage regulator. When I finished installing, voltmeter showed 14.5 volts running. Later in the day, I started getting readings in 15.5-.75 ranges. Ammeter gauge does not work(??). What gives? Truck has a 1995 5.9 magnum motor with the 120 amp alternator. Thanks in advance for the advise.

while it is charging high, you will have to check some voltages to verify what is causing the problem.
with the key in run but engine not running.
you will want the voltage at the battery using the battery ground and battery positive.
voltage at the voltage regulator going into the voltage regulator using the voltage regulator body as ground
voltage at the voltage regulator going into the voltage regulator using the battery ground as ground.
voltage at the back of the alternator big post using the battery as ground.
voltage at the alternator both small posts, but you really care about the one that gives battery voltage using the alt case as ground.

generally you should find a drop in voltage somewhere. where ever it doesn't match battery voltage, you need to back track and check voltage again at each connector. eventually you should find a bad connector.
OR if you get different voltages using the case as ground vs the battery as ground, then you need to add a ground to that case of the VR.

I hope you are not using the cheapo harbor freight voltmeter, they are notoriously inaccurate.