Double check the ground connection FROM the battery to the BODY/FRAME...Early ES's had developed a similar issue. It is caused by age/enviroment/material breakdown and you can guess the rest...
What I am advising is to disconnect the contacts and clean them and reinstall, Or better yet replace the Neg cable with huge guage copper cable (not wire, My guage numbers escape me at this time, I beleive it is 10 guage or bigger)