Alternators do not put out max amps at idle. If your demand is more than the alternator can output, then the battery takes up the slack... for a little while. But the voltage will drop under heavy demand at idle, that's normal. The more current you demand, the more the voltage will drop.
Also, you need to understand that an alternator has 3 windings, each producing 1/3 of the alternator's rated output. If one winding goes bad (blown diode) the alternator will continue to produce 14 volts, but only 2/3 of the amps. So if you're asking more amps than the alternator can supply, the voltage will drop.
So first, get your alternator tested.
The road goes on forever, the party never ends....