I agree with R Pope. If your battery is grounded to the body or the frame, make sure you have a ground strap (cable) from the engine to the body or frame or, better yet, run your ground from the battery direct to the engine block if possible. It sounds like your starter is seeking a ground through whatever route it can find; it will eventually start melting wires...

You can test this theory by hooking an ordinary jumper cable from the ground post on the battery to somewhere on the engine. Make sure you have a good connection; then crank it and see if the gauge still does that. Let us know what you find out.