Can you run a simple test for me please? Select both pickups, dial the neck volume back to approx. 8. While strumming Does the Bridge volume work normal all the way up and down?
Now dial the bridge back to approx. 8 and turn the volume up and down while strumming and tell me if it works ok now?
Let me know please. I am trying to figure what is happening. I suspect one or both pickups are affecting the other.