At idle, it is normal for alternator output to drop when load increases. As RPM increase, the alternator output goes up. Although you may have a 70A rated alt, you will almost never see an alt hit its peak out in the real world. Mainly because most of the current draw is coming off the battery, battery level starts to drop, alternator senses this and increases output. The battery acts a filter for the load, rather than the load the going straight to the alternator.