Electrical problems. Battery or alternator?

I have a 93 new yorker. About a month ago I was driving and out of no where my car started normally but began stalling then surging all within a second or two of each other while I was driving. I looked at all of my gauges and the votage gauge was jumping up and down spuratically. After completing a 30 min drive I turned the car off and didnt return until and hour later and the car started normally and was no longer stalling and surging, but I did nite that the voltage would shoot up and return to normal. After that day my car would start but soon afterwards the voltage would drop and shoot back up everything would dim and the engine light would flash on and off. Once driving the car would stall and at stop lights or while idle would stop running completely engine light would come on and go off and the check gauge light would remain on ( particuliarly worse at night with headlights on). I would restart car and continue driving it seems that as long as I was accelarating on the gas I was pretty much ok and the longer I drive the less frequent the syptoms occur. I had the alternator tested I was told it was good and that it was putting out 9 volts or amps.I cant remember which of the two. The person testing the alternator noticed some corrosion on the battery and told me it was the battery and that the corrosion may even be in the wires. There was some corrosion but I have even seen more with a proevious battery I had tooken out the year before. Utimately I had the battery replaced it was tested as being bad. But Im still having the same problems with the new battery. What could it be?