MTL
Don't know if you have found your problem yet but here are a couple of things that might help.
The oil tank on the 67 is pressurized to about 5 - 7 psi higher than the AGB - the pressure is controlled by the "Oil Tank Pressurizing Valve". It is possible that you have a problem with the valve and at higher power settings the AGB is being over-pressurized.
Another possibility - unlikely - but I had it happen to a - 42 engine - is excessive flow through a labyrinth seal that allowed too much air to be dumped into the scavenge system - resulting in an over-pressure of the oil tank and AGB as the oil tank pressurizing valve could not cope with the increased flow rate.
The last possibility that I can think of is a restriction of the overboard vent that is not allowing the discharge air free flow. I know that you stated that you checked the line - but the fact that the leak stops when the line is disconnected tells me that the problem is associated with the vent line.
It is a long shot - but try a different length of vent tube as you could possibly have a standing wave backpressure condition - think of tuned exhaust theory.
There must be a tap point in the system that you can connect a pressure gage to the AGB / Oil Tank so you can get a pressure reading - that might point you in the right direction.
If you do find the problem - or have found the problem lets us know - it adds to the information bank.
Regards,
Mx