[olddog]

Steel is not the best conductor, especially when high amp is drawn such as a starter. Low current stuff doesn't matter for auto stuff. It's not good to ground the battery to one end of the frame and the engine to the other. That's a long distance to pull that many amps through steel. A really good quality jumper cable from the battery negative to a starter bolt should be a good test to see if it makes a difference.

If you are loosing a volt or so through the conductors (frame), the starter motor will pull more amps to make the power needed to turn the engine, provided the battery will supply it. The higher the amps the hotter the starter gets. Over time the extra heat may be take its tole.

I have noticed people will run plenty heavy wire on the positive and then overload the negative. The negative side doesn't seem to get much thought, but what ever current is flowing on the positive wires there is an equal amount on the negative.