sound like heat soak in the starter or the starter solenoid. could be the resistor wire in the ignition. Its a built in ballast that when it gets hot it increases the resistance and makes starting harder. thats why when you jump the solenoid by it works. To stop this from happening, find a wire that gets 12volts when you try to start only, then you can use this to bypass the resistor wire for starting, but when it running it will use that resistor wire so as not to blow up the coil and ignition.