Voltage falls for no reason :-( Battery old or Alternator goes south??? :-(

[sobi6111]

i got inside my garage today and opened the trunk and so all interior lights were on obviously,and engine was still running and xenon lights were on and music wa splaying and etc. Well anyway,after 2-3 minutes i noticed like lights became darker and then in a second they got back bright. I was liek WTF. Then in another 2-3 minutes it happened again,so interval 2-3 minutes. It feels liek when that happen every2-3 minutes or so,it almost makes engine to stall,but then he gets rmp higer and then lights go back to normal and etc. Any ideas???? What coudl that be?