I had the same problem with my Khyber.
The alternator's voltage used to drop with A/C and headlights switched on at the same time.
I checked with DMM and the voltage was as under:
Overnight Battery Voltage: 12.4V
At idle: 14.28V
With headlights only: 14.07V
With A/C only: 13.7V
With both above: 11.8V (this is very less, the voltage should not drop below 13V under all load)
I replaced the alternator to battery cable, installed an additional earth from battery negative to alternator mounting lug but nothing worked. Then I replaced the alternator with a bigger one having more current output. Actually, the voltage drops when more current is required.
Now the problem has resolved completely. Following are the new readings.
Overnight Battery Voltage: 12.4V
At idle: 14.35V
With headlights only: 14.2V
With A/C only: 14.05V
With both above: 13.8V
Maximum shoot with no load: 14.41V (This is done to check the alternator is not overcharging the battery)