Probably the printed curcuit film on the dash cluster making a poor connection to the gauge,or the gauge itself.....I had an El-Camino that showed a discharge at times,so I replaced the altenator with a used one I knew worked good I kept from another Chevy I had---did the same thing....I tried another one in case that one went sour from sitting,no difference............................................................................................the battery never went dead,so I decided to ignore the gauge......then one day my brother reached in the cab when I complained it was "discharging" again,and he pressed firmly on the plastic over the volt gauge,and it climbed right back up to about 14V!...the gauge evidently wasn't making good contact with the printed curcuit film on the dash cluster....I "fixed" it by shoving a penny between the plastic cover on the dash and the cluster,to keep a firm connection and it never acted up again...
