I'm having a similar issue going through alternators. I've owned my 2000 f250 for about 3 1/2 years and have put 5 alternators in it. I just installed the 5th today so thought I'd search and see if anyone else has this issue. Bought the first three at advanced auto and they only lasted a couple months. The last two are from napa and lasted about a year or so. Doesn't make sense to me that I'd be running through alternators like this. If anyone figures this out please let us know.