ASFKAP
Its possible that at a certain power setting one or more of the earths is breaking down due to a combination of vibration on an under torqued connection and or contamination or corrosion of the joint which might not show up under static conditions.
Totally agree, almost by definition any bonding fault (assuming that that is the root cause) will not be found by a simple test on the ground since the problem only shows with engine RPM above a certain level.
Even a sensitive (mv) test may/may not find it although it has a better chance.