Sounds like a loose connection, positive or ground. When problem is ongoing, jump starter relay, load side, the load side is that line with arrowhead, unplug relay, use jumper wire between those two terminals on left side of diagram. The starter motor should crank. If you try that make sure tranny is in park or neutral and parking brake is set. If it cranks with jumper but won't otherwise, have to check control side of relay, right side in the picture. That top wire is voltage from ignition switch, bottom wire is ground.