problem charging system

[catahoulasr1]

i have a 96 powerstroke with two different batteries one from car quest one from auto zone. i've replaced the alternator 4 or 5 times now in the past year and a half three days ago the alternator went out again. both batteries test good both at about 12.2 volts put the new alternator in about three days ago. can charge the batteries and will start. coming out of the alternator it tests low showin 12.2 should be testin somewhere around 13.5 at idle right? you can get in the pickup and watch the voltage meter and it is constantly movin back an forth. which is definately not the norm. so batteries test ok new alternator already looks bad. cheap alternator? a ground somewhere? but something seems to be pullin from somewhere killin alternators.