Intermittent grounds are very difficult to resolve as they can only be troubleshot when the ground fault is active. The manner in which to find the ground is the half method. Remove the circuit from the Fire Alarm Panel and locate a device in the middle of the circuit. Test for the ground fault and determine which direction the ground fault is coming from, either toward the fire alarm panel or towards the end of line. Continue to split the effected circuit in half until you find a grounded device, or circuit section that will require replacement.