The ground cable might be corroded inside, causing excessive resistance. Using a low-reading voltmeter connected between the starter housing and the negative post of the battery during cranking check how much voltage is being lost on the ground side. It shouldn't be more than 1/10 volt.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!