No, I'm not using MySensors on the Arduino, I'm using RadioHead. I posted on here as there seemed to be a lot of people using that module on this forum. But yes: the radios are both configured to 2Mbps. The fact that the RPi can receive even though not transmit certainly lends weight to @mfalkvidd's suggestion that it might be power-related. I'm getting some voltage regulators and will power it separately from the Pi to see if that improves anything. On a related note ... why are 3.3V regulators so hard to come by!? It's been years since I last picked up a soldering iron... Grumble