Luigi welcome aboard! & you have just made contact with the best/sharpest/craziest bunch of Mopar addicts in the world. You have the electronic reg & they either go open or full field & when they do they stay that way so I would opinionate that you have a part time open (not in the reg) that is dropping the voltage cuz it ain't charging then & then when it makes contact it bumps it up high to restore the level which means if that is the case it would be dropping to 12 ish when it is in open mode HOWEVER 17/18 is real high and you might be dealing with a PT dead short in the green wire circuit including brushes/brush holders/alt internal field circuit (got another alt you can sub in for a quick test?). this must be fixed ASAP as 17/18 volts will fry electrical devices onboard in short order (bulbs will go first).