I can tell you right now that your problem has to do with drawing too much current somewhere. When there is too much current through the relay, it does this. It could be the starter, it could be the connection to the starter, it could be the ground connection to the motor, it can be the connections to the battery, it can also be a bad battery. I've had bad batteries cause this. What happens is there is not quite enough voltage to turn the starter motor (either due to bad battery or bad connections), so it draws more current to compensate, which welds the relay contacts. First and foremost, make sure your connections to the battery, starter, and ground to motor are solid as can be. These are all the big cables. Make sure there is no corrosion on these connections. Make sure there is no paint between the ground cable lug and the motor. Take a wire brush and clean between all of these connections. Make sure the cables themselves are solid all the way through.