Alternator problems?

Ive got a 2001 montana that is having an issue with the interior and exterior lights dimming and brightening when idling. I opened up the hood while it was idling and noticed that the belt tensioner is moving slightly. Would this affect the alternator while the vehicle is idling? Once I press on the gas and the belt spins a bit faster everything is fine.

Is the belt skipping on the alternator and causing irregular currents while idling?

Slight movement of the idler is normal, have or test the alt output to see what is happening.