My Voltage gauge moves back and forth from 14.5 to 13.5 when sitting at a light with nothing else on. I can hear the lift pump changing its noise with the fluctuation of voltage.

Then when I turn up my stereo loud and even when I am moving the whole dash dims almost completley and the scan Gauge sometimes goes to 12.5V

It is charging but maybe the voltage regulator is going bad.

Do I need a new alterantor?

On the scan gauge 13.2-14.2V seem to be the fluctuation I see without the stereo on and with the stereo I see 12.1-15.1V

Could be anyone of the electrical components. Just have to isolate and test.

Do the Simple thing first. Check both Batteries' Connectors for any Corrosion, and make sure they are Screwed in to the Battery nice and tight.

all the time or only when cold?

All the Time

Also The truck has brand new optima Yellow tops

GROUNDS GROUNDS GROUNDS, clean em!

mine does the same thing but only when i come to a stop.

volt gauge shows about 14. when i put on the blinker it goes down to 12.5 or so and - up and down w/blinker on/off. the WIF light came on yesterday for a few minutes till warmed up (just bought truck, need to change filter saturday), when the blinker came on, it would flash on/off in time w/blinker.
voltage regulator or grounds? yes, i will do the grounds this weekend.
jon

Take off the battery cables one battery at a time, clean the connections, test the battery while unhooked, rehook battery and repeat for second battery. When finished load test the alternator.