1997 Ford Crown Victoria Question About My Voltage?

I Have A 1997 Crown Victoria and im having a problem. The other day I was driving home from work I noticed that my voltage gauge on my dash was pretty much down at the bottom of the gauge showing low. Then what happened seconds later was that my dash lights had gone out and I had no lights at all. My radio didnt work and my spedometer stop working. All of this had happend when I was on the freeway doing 70 MPH. I drove for about 8 miles like that until I came to the offramp and stopped, then my car had died. Me and my friend then jumped it and everything was fine. We then took off the jumper cables and drove for about 200 feet then everything blacked out again. Luckily home was only a couple of blocks away so made it but as soon as I braked to stop the car died. Now, before all this. I had put a NEW battery in the car and had the Alternator checked, and it was fine (14+). All the wireing looks fine. And I dont know whats wrong. Any Ideas?