1990 Isuzu Impulse 3 lights

I had the three lights come on replaced the altenator it worked good for a while and then the problem happened again. This time I replaced the battery and the altenator it ran well until I started the air conditioner the lights now com on again. When they com on it seems that there is someting causing a large drain on the system the radio goes off the heater moter goes off the lights dim. Reving up the moter somettimes works but sometimes the lights stay on I took the car to autozone to be tested they said the alternator and battery tested good but the lights were not on the. Itried to make them come on but of course they would not. I thought the air conditioner was comming on so idisconnected the compeeror I also disconnected the air conditoner cooling fan istill have the problem could the altenator be going bad because when I put the air conditioner on I started to have the problem I am about to buy a new altenator but this will be the fourth one and it is getting expensive