2000 626 V6
Driving along and alternator light flickers. Sometimes comes on for 5 seconds. It can happen 10 times a week or only once a month. It's been happening on and off for over a year now.
I've checked the battery and cleaned the terminals but thats it so far.
Is the alternator going bad?

usually if it is the alternator its gonna go out and wont return. try changing the belt or tightening the tension on the belt for the alternator.

Mechanic says the belt is worn and should be replaced. I just had it replaced three years ago. Could this be the problem and so soon? I thought belts were good for at least four years.

if the belt looks worn or cracked it needs to be replaced. if not maybe the idler tensioner needs to be replaced.