First off Y connectors are just bad news. Basically you are hooking up two potentiometers that will never be in sync to a single pulse. That creates a sort of feedback just like a microphone too close to the speaker. Some servos can handle the noise some can't. Not sure why you must have a Y as just about every modern radio system has dual aileron servo mixing. Having your TX too close to the RX or setting up in a confined area WAS an issue with 72mhz. 2.4 does not have those issues. I suspect that the ESC BEC is putting out different voltages. It's possible the first two were putting out too much voltage, I have seen the same thing myself when I tried to run Futaba 4.8V servos on 6.6V.