Im needing some help with my truck i recent acquired and appreciate any insight on this situation. The truck has a new battery and alternator and both were test a few days ago to be sure their good. The voltage gauge sits in the middle of the battery picture sometimes, which seems normal. Other times it sits just below the corner of the battery on the low side. If i turn the headlights on it temporarily plummets then raises back up. Sometimes the truck will die and it will barely turn over like the battery is dead and when it starts up the gauge reads real low for a minute or two. The problem is much more common at night with accessories on like the lights, radio etc. The belt appears in great shape, I wouldnt think its slipping anyway no belt noises. The terminals are clean as a whistle and tight has can be. The battery is a new optima.... Im really unsure where to go with this problem any ideas?