sun super tach ll question for ya

i have a new SST2 and a MSD blaster coil . i have the tach set at 8 cylinders , the red at hot and the black at ground and the white is not connected up (face light) and the green is on the negative side of the coil.when i start the car , the needle idles at around 700 RPMs. when i get in and go the needle drops in RPMs and comes back up when i slow down.WTF ? its like it is working backwards. is my tach jacked up? am i missing something here? tachs are a no brainer but this thing has me stumped.would it act like that if the ground and hot were switched? it seems to me it wouldnt work at all.is it a bad ground? or can i not use it with the MSD coil? i know this sounds wack but im at a loss lol .any help would be great thanks.