1996 Ford Probe SE, 2.0 L, 4 cyl., 128K miles.

I recently had to replace the alternator in this car myself, also replacing the battery and belts. I made my bi-annual drive from college in Florida to upstate NY where I live, about 1100 miles, and everything was fine. However, now that i'm up here, the car's guage showing battery strength is all over the place. Sometimes it's on the high end of the normal spectrum, and other times it drops down to the Low marker almost completely. I have no clue what is wrong at this point, any ideas?