New alternator failed after 10 mins.

Just slapped in the new alternator after the last one started showing signs of going out. Started up the car and it held 14V for a maximum of 5 minutes, then it dropped to 13.8, then 13.6 and then it just stopped charging all together. All in the span of 10 minutes. It took forever to find this alternator and given my location and the type of engine I can't just return it or get another one anytime soon. Just don't get how a new alternator works for just 10 minutes, can anyone share some insights please, did I miss something? All the fuses, wiring, connectors, etc. are all perfect and triple checked, I don't know what else to check or do apart from just slapping in the old alternator again, at least it charged somewhat.