Is your battery light on? are you checking voltage at idle or about 1500 rpm? If the plug firmly seated in the alt? are pins bent in the connector? Another thing to consider is bench testing for voltage is ok but it must be load tested to know if the alt is giving up on life or not ( a good reg will set voltage at 14 but alt might be in poor condition and only able to muster 12)

__________________
1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car