Originally Posted by blackoval86
I would think the alternator is taking over once it cranks. Maybe it's not putting out enough amps at idle?
That could be. If the alternator is dieing the battery will have to work harder to keep the voltage correct at the battery.
Before you start the car, check voltage at the battery. should be right around 12 VDC. With the truck running the voltage at the battery should be between 13.8 and 14.4.
Under that the alternator is bad if it is internally regulated. If externally regulated you can replace the voltage regulator first to see if that helps.
Over 14.4 is bad too as the alternator is at max output 100% of the time and will shorten its life. But I think the 1st scenario is a better fit.
If the voltage is below 14.4 then the ignition will not work correctly and run rough.