Check your grounds... ALL OF THEM
ALso check that the negative battery post clamp is not loose or over worn...
Grounds to check will be negative cable to top of transmission, cable from valve cover to radiator support, and also the small black clip wire to the top of the starter, & the groud from the engine block to the front of the car.
My money ois on the negative battery cable. I had the same issue & all grounds looked good, but the issue was fixed by actually replacing the damn terminal clamp on the negative cable...