head scratcher!!!

OK ground strap from block to firewall is there. Cranking amps ( good battery ) is there.
Heres one , may not be related, then again....
Check and clean away some paint at battery ground cable to block. Some fox body mustangs came off the line with the star washer between the wire lug and the block. 6 tiny points of contact. One time it starts , next time it dont.