What kind of panel are you working on? I've had problems with Edwards panel and so-called "ghost faults". Basically its a very high resistance ground fault, the technicians have ended up installing 1 megohm resistors attached to ground to create a drain, very similar to what you do with a shielded cable on fire alarm. These faults would disappear for days at a time but after making contact with Edwards it was a common problem and easy fix.
