Two Bluetooth devices + MacBook Pro + 2.4gHz WiFi = no network throughput; why?
I had the same issue. Did a bunch of troubleshooting with Apple over the phone and in store. The final diagnosis was a logic board and wireless card replacement.

When they took it in to repair it, the tech noticed only one of the three antennas were connected to the motherboard. His theory is that the one antenna was enough to support a connection or two but became overloaded in certain situations, very easily. After reconnecting them all has been working well.

So my suggestion to you is to either open it up yourself or bring it to Apple and see if that may be the issue.