2000 Ford Focus

I have a 2000 Ford Focus, yesterday I was driving it down the High and
the radio went out moments later the instrument cluster died and the
the car Started running funny. I pulled over and cut it off. Then it
would not start, so I had it towed back to my house. I took the
battery up to the local auto store and had it tested, they said it was
fine after they charged the battery I took it back to my house and put
it back in and he car started. So i drove it up to the same store and
they check the alternator and it was fine. The only thing that I
noticed was a little bit of corrosion on the batery terminal. Any one
have any ideas what may be wrong with it.