from experience on other other equipment, the starter relay issue is either dirty connections, wiring beginning to corrode inside the insulation or too light of a wire gauge. too much resistance will cook relays and solenoids pretty quickly. If all the connections are good, i'd put in a clean run of new wire from the battery all the way through the circuit and consider stepping up to the next largest gauge size. If that doesn't do it, then there's either an issue with the solenoid or the starter that is causing there to be too much resistance and overheating the circuit.
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