You probably have missing what many C24 mic systems have missing: a comprehensive, thorough grounding and shielding scheme. AKG was notorious for this in its tube mics, especially in the C24 which took the price for non-sensical (and constantly changing) grounding and shielding.
Bottom line: The entrance point for RF is somewhere where the shield is interrupted. You need 100% shield coverage from head basket to XLR shell at the input of your mic pre/mixer. An Ohm meter will not show this, only thorough visual inspection of ALL connections, from the mic's cable connector to the final XLR, all housing parts of connectors must be connected to ground AND cable shields. NO exception.