tach issues

hay guys wondering if anyone had this problem I have a brand new Stewart warner digital tach and shift light when I'm doing a pull the tach will start to not read accurately at the top of a pull around 4600 or so what it does it will read 4600 then 5200 back to 4300 and just bounce around and the shift light will come on and off and back on the car doesn't miss of stall its smooth I have a recurved stock mopar dizzy I have it converted to a gm hei ignition module from Davis unified and and a msd blaster ss coil and I have the tach input hooked up to the negative side of the coil any help would be great and I also had this problem with a different tach I had in the car