What causes battery light to flicker???

This has been going on for the past 2-3 wks or so and just noticed today that my battery meter, when I slow down from doing 40-50 mph, that my voltage jumps up and down....what the hell is goin on???? Do I need a new battery already?? The battery I currently have is a Costco brand and is prob oh...2 yrs old at the most. Now, the battery light on my instrument cluster isn't on constantly at all...just at certain times and flickers whenever it wants.