i run a 1 inch SS braided cable from block to frame and a 1/4 in one to the body .
one thing on a car that sets is cables corrode over time. i just replaced the cables on my f250 and cut the sheath off the old ones. lot of corrosion inside . seems common on diesels .
at low rpm you should not drop below 12v unless your running a lot of high amp stuff . might check your battery also .