Nissan Repair: Electrical 97 nissan Sentra gxe, nissan sentra, altenator

Question
First of all my battery kept crusting up on the postive cable to the point of needing to be replaced. So it was replaced but I was still having problems with the electrical system. As I was driving with the air and the radio on my speedometer drop out to zero and I was still going about 55 I also noticed the airbag light flashing. When I turned off the ac and the radio the speedometer came back. I thought I should go to auto zone and let them check my altenator and they said it was bad. I didn't really believe them so I replaced my battery and my car done fine for only a few days and then the problems returned again. The air bag light was flashing again also the battery and brake light remained on. My son took the altenator off and we took it to auto zone and it passed the clerk tested it again and it still check out to be good. What could be wrong? Please help me. I am a kind of do it yourself kind of gal so please explain so I might DIY.
Thank You So Much..............Janet Calvert

Answer
Janet,

Check the cables that connect to the battery for corrosion and to be sure that all the connections are good and tight.  But, not too tight.  Also, I would go to another parts store and have them check the system while it is running to make sure that you are getting proper output from the charging system,

Now, with that said I would also look at the ignition while the car is doing the cut off thing and wiggle the key and see if it changes the situation.  Basically, we are looking at an electrical problem here and one that is intermittent.  So, the problem can be one of those mentioned and I would look in the order stated.