If it still has the neutral start switch, that's a place to start. On the D
series it's on the right hand side of the shuttle case towards the top. Should be two wires going to it. I dunno if the C and D are the same but that's where mine is. Could also be voltage drop to the start solenoid. Had that happen on a different make tractor. Verify voltage to the "go" wire on the solenoid and trace wires to the neutral switch.