BONESZONE73

Hello all, I've got a voltage issue with my 1994 F150 4x4, with 4.9 engine, I started noticing my voltage gage jumping around last week, I live out in AZ, so I was sure my battery was getting ready to expire, since it has been over 3 years since I replaced it, well I was able to finally work on it today and I found a bad negative post terminal, I replaced the terminal and since I had the battery disconnected, I replaced that too, I fired it up and voltage gauge was still jumping around, so I pull the alt. off and had it tested at 2 different local parts stores, it showed the alt was good/passed, so I reinstalled it and now the gauge doesn't jump around, however when I turn on the lights and the a/c, voltage drops to below 10 +/- a few. I checked all the wiring for obvious shorts, but do not see anything, so now not sure what direction to go. Do I replace the alternator even though they said it was good? The truck has 206,000 miles and it's never been replaced, or do I try a voltage regulator? If someone could point me in right direction it would be appreciated.