alternator issue??
 

I have a 1990 3VZE 3.0L V6 automaticwith about 150k on it, recently has begun to do the following with no discernable pattern: 3 dummy dash lights (e-brake, battery and a/t oil temp) will light up, at the same time the battery voltage gauge will drop to the lower operational range (not to zero but to the left "within limits" line). The engine continues to run just fine. This is an intermittent issue that seems to happen randomly. I checked the batt. with a voltimeter while this issue was occuring and the normal charging range of about 13.5v drops to less than 12, like 11.5 or so. Has this happened to anyone else and does anyone have any idea what this is? Thanks folks!