driving in stop and go traffic...CD players starts flickering turning on and off like it's posessed...check battery voltage...usually around 14...now sitting around 12...then drops to 8...lights start dimming...start driving shoots back up to 14...stop...dips to 12 then down to 8...is my alt belt slipping...???

 

or your alt is just starting to sh*t the bed. My guess is that it would slip more under load in gear than not. Try a can of belt dressing and apply it to the insides of the belt wheels with the car idling. If that doesn't work, check the belt tension and a new belt. If not that, then... probably $$$.

 

anything under 12.6 volts means the alternator is not producing enough power to even run the car and you are running on battery power.

belts normally don't slip at idle, but rather at higher RPMs. Your alternator is failing