I have an orange box Mopar performance system and my tach was very erratic, sometimes not working. Sent it out to auto instruments who found the tach to work on the bench but was not compatible with the orange box. They replaced the board with a modern one that is compatible with all systems. Tach is very accurate now.
The factory tachs are not very accurate anyway. I have done this upgrade on a couple of Rallye tachs, easy to do... RTE 67-74 Tach kit - rte
Also you should replace the voltage limiter to the gauges while you are at it.. the original is a piece of dung! RTE limiter - rte