Can you be sure it's not the earthing points then? The starter earths through the engine rear plate, which is connected to the body by the earth strap above the left engine mount (behind the oil pump). One end fits onto the stud in the block (which comes through the side plate so there could be surface corrosion). The other end fits to the securing bolt for the engine mounting, which could also corrode from underneath, where it is open to the elements. After that, check you're earth strap in the boot - it's only a flimsy little tab that secures it to the body.
If any of them get very very warm after cranking it's a sign there's a dirty joint somewhere.
Removing the straps and cleaning the contact surfaces up with some fine emery is a very good idea. Then smother them in some vaseline when they're back on. Then do a continuity from the engine to body, and body to battery. If they as near as dammit 0, you may need to clean up around the starter where it mates with the engine rear plate.
Michael
