weird charging/alternator problem

i have a 94 buick century and having some problems with the alternator or some kinda wiering problem i think. this is gonna be my 3 alternator this month i dont really know if its that alternator again or some kinda wiere loose. some times it starts fine and i can see the voltmeter charging and some time i see it below half and lights go dim i accelerate and gose back up

Check your battery terminals for good connection and check your grounds.

Also check youre serpentine belt, tensioner, and other belt driven accessories for any lateral free-play or evidence of bearing failure.