General Maintenance & Repairs - Lights go dim

Did a search but not much info. I have a buddy with an 00 ss and a problem just started. The voltage light is fluctuating while hitting bumps N the dash/radio lights go dim. And idea? Altinator? Terminals are tight. Grounds look good... Anything will help. Thank you'll

Could be a bad battery?

That's what I was thinking too. I would understand that if it just wasn't powering up at all. But hitting the bumps is kina weird

Ok so update. It only happens when speed is right at 60 mph.... Any higher or lower it doesn't do anything