1985 Nissan Truck Changing alternators a lot......

I own a 1995 Nissan King Cab truck. In the two and a half years that I have owned it I have been through three alternators. I believe that this is not normal. I seem to have problems in the electrical area with this truck. It seems to be happening around the battery/ alternator area. When the alternator goes out it happens slowly. First, through the radio, it sound like the people on it are under water a gargling sound. I have to push the gas harder to get it to go. Using the blinkers or lights makes it worse. It will run like this for a few miles then stop. I guess this is due to the alternator stopping and the engine running off the battery. I change the alternator every nine months or so. I don’t know if this helps but when listening to the radio on AM I can hear the alternator hum on it especially if the station weakens.