removing the alt from the car to "test" it is retarted. alot of alt probs are heat related. it may charge 110 amps at 14.5 volts when cold but then it get hot and...

simple but not 100% is to just put a meter on the battery with it running and check the voltage.
100% way is to run car at 2k
apply high load with tester (to a good battery)
measure amp output and voltage.

since it is sporadic checking cables/connections is a def first like kirk said