I agree with both John G and Whizmo. Check the terminal clamps on the battery post.
When I did my most recent door mod I had to disconnect the battery.
When all was done I could not get a good tight fit on the battery post.
I would have never caught this, if not for doing the door mod and this could have
definitely lead to intermittent starting problems. Possibly even failure on the highway.
I used washers to fix the situation quickly but they also make battery shims that fit
over the posts.
Hope this is the problem your having.
