battery guage

has anyone ever had this happen.it only happens at night when the headlights are on but from time to time my guage will read like 14-15 volts then get a surge and go down right below the 13 mark then back up.when this happens everything goes dim.im worried cause i wanna put a sub and amp in and dont wanna have a huge problem

My car does that sometimes, even during the day. Thought it might've been the air pump cycling so I removed the fuse - still did it just not below 13. I don't have a system and Optima red top is only a couple months old. :thinkin: