I have a 91z28 with a t56 hooked up. I am running the dakota digital SGI-5 box and when the car is idling in neuatral, the speedo needle is running between 50-120mph. When I turn on something like the wipers or rear defrosters, the needle drops to 0 real fast and then goes back up. it works fine while I am driving but as soon as I stop its wacky again. what gives.