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just recently my 88 f350 with a 460fi has started to not want to start well. Sometimes it acts like the battery is week then starts. And other times it just cranks untill i move the cables around. i checked and it is getting fuel and spark each time. I cleaned the batt terminals and cap/rotor.
Note. i did have the starter,etc off last weekend to do a oilpan gasket but ran fine until yesterday(Thursday).
Replace all the battery cables. Any decent parts store will have them. Don't get them from a junkyard.
The plastic casing on the cable breaks down over time and allows moisture into the cable. It corrodes and doesn't conduct enough electricity and you get what you're describing. I would also clean all the cable connections when you do this.