What amperage capacity relay are you using? A 30 amp might be on the ragged edge of too light duty, as the initial current surge of that big solenoid is pretty high before the inductive reactance of the coil kicks in. I'd suggest going to a 60 amp if you decide to replace it. Yes, they can get intermittent when the points start to fail. I'd also suggest connecting an ignition condenser between the solenoid hot terminal and ground to protect the relay points from induced voltage and possible flashover as it engages and disengages.
Jerry

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