Your battery cables may be corroded inside.
A few months ago I was having running problems and some funky electrical stuff going on, new negative cable.
Last week I has no start problems, new positive cable. For the positive cable I would drive it fine for a awhile, then nothing. I just happened to move the cable alittle and everything was fine, so I knew it was corroded on the inside. All week with the new cable, everythings good.