Bridge mode speed issues

I have a gigabit connection, and if bridge mode is disabled, I see close to gigabit speeds (914/43).  As soon as I enable bridge mode and restart the modem, the speed is significantly reduced, getting around 500/43.  As soon as I disable bridge mode and restart, the gigabit speeds return.  The only other thing that changes is my external IP address: bridging disabled = 73.172.x.x / bridging enabled = 69.250.x.x

Any ideas what is causing this significant speed drop?  All downstream equipment remains exactly the same in both configurations.

Thanks in advance.