Noticed my factory alt/ batt gauge was doing some crazy stuff during a 100 mile drive home. It has historically always read low, somewhere around 13 volts however this trip I noticed it was uncharacteristically high for a period, then a sudden swing to very low which also trigger the battery light, then back to the normal 13 volt range where it traditionally is. Any opinions what this could be? Wiring issue? Alternator on it's way out? Voltage reg? As a side note, this is a completely stock factory system never been modified. Thanks in advance!