You should be able to maintain a constant 14volts give or take .2 volts with a fully charged battery.
I would be willing to bet you have a couple issues which makes it more difficult to diagnose.
First you need a good multimeter to see what the voltage really is at different points in the system.
You need to pull the battery and have it charged and tested. Recently I have seen batteries cause weird issues.
You need to be sure of exactly what you have for an alternator. A one wire is a waste of time in my opinion and will not necessarily work properly when the other wires are connected. The voltage regulator is the main difference between a one wire and a three wire.
There is a small d shaped hole in the back of the alternator. In that hole there is a little metal tab. If you carefully insert a small screwdriver into that hole and ground that tab it will force full output on the alternator. This may help you test it.
Artificial Intelligence is no match for Natural Stupidity