Edit: I was just looking at your pictures. Source the MBC air supply from a different place than your BOV. When air is bled out to your wastegate from that line by your MBC, it creates a pressure differential between the top and bottom of the BOV letting it open. Basically you are having the opposite of compressor surge. Your BOV is opening too easily as opposed to not easily enough. This is why lowering the boost to close to the pressure threshold of the WGA spring helps get rid of this problem. When the system is closed such that no air is making it to the WGA, your BOV is equalized. However, the more pressure that builds, the more the valve tries to hold the wastegate open, the more air is bled away from your BOV, and the less pressure it has holding the valve closed. Then when the wastegate opens, less pressure is made by the turbo, so less pressure is pushing the BOV open, so then the air can start going to your engine again, which makes more boost, repeat repeat etc. However, as boost comes on even harder, the manifold pressure is high enough to supply that little line with enough pressure to hold your WGA open and keep the BOV pressurized, not to mention the WGA pressure is not fluctuating as much as it was during transient boost levels, and is now holding steady, so there is less need to vent air back through the MBC while opening-closing-opening the wastegate. Remember at part throttle the throttle body butterfly is slightly closed so as the engine revs there is likely to be fluctuations in the pressure differential of the air on either side of it.

Hope that helps!

Edit #2: If what I said above isn't obvious, here is what I recommend you do:
Run a dedicated line from the Intake Manifold to your BOV with no T.
Run a line T'd off of another line running off the intake manifold to your MBC, or drill a nipple into your lower intercooler pipe and use that air supply.
Problem solved.