Hi.
When you have the problem, is the RPM increasing or remaining constant? If the problem was a reduction in oil supply to the pitch change piston, then the propeller will tend to fine off and RPM would rise unless you were already on the fine pitch stops, which is unlikely. If the RPM doesn't rise, and the prop was already coarser than fine pitch stop, then the problem is not related to oil supply, it remains with the governor.
Your process of changing out the governor is sound, but as the fault remained then the problem if not oil supply has to be from a loss of input to the flywheel of the governor )it is a simple mechanical device... not much magic there) If the RPM is not following the RPM control changing for steady speed conditions, then the position of the output arm of the governor has to be fixed, and that probably means that the drive to the governor has a failure such as a fracture that sometimes slips and at other times locks in place. It is conceivable that both governors are defective, but if the replacement or your works correctly on another aircraft, then that would remove the governor component as the cause, and only leaves the input from the engine to the governor as a source of the problem.
Your contacts are the right ones to be talking with, but they probably need to observe the problem first hand to really get their head around the matter.