Loss of power, battery draining leading up to severe miss firing and back fire

Problem came on suddenly.  I made no changes or additions to the truck.
Then the voltage needle dropped from center at about 14 to less than half that.  Had the battery/alternator checked they said the battery was bad.
Same problem occurred.  Called my mechanic he suggested replacing the alternator.  I did with a new (not rebuilt) one.  Same problem occurred.  One thing I forgot to mention occasionally maybe one out of five times at the beginning of this the voltage needle would go back to normal about 14 and the truck would run fine.  But recently it has not gone back to normal and battery is so discharged that I have to jump it or put it on a charger to even start it.  I am a do it yourselfer  so any suggestions?

Thanks

Mark