Maybe might be the alternator? It happened to me before.
Here is my situtation:
(237,000 Miles) 4.0L Inline 6 Cy.
on 500 Mile trip towing boat, go over mtn pass, spend the night at a friends house, leave and 100 miles into the next leg of the trip, my check engine light goes on and my voltage drops dramatically. Vehicle still starts and run great. On and off my engine light goes and my voltage jumps from 9-14 Volts throughout periods of the trip. Was driving today and it dove down to pegged the 9 volt bar and my vehicle started to sputter and almost died going down highway.