Hi there,
The snow effect is caused by a handshake issue with the HDMI connection through the onkyo reciever. It will probably occur via any other HDMI component attached via the Onkyo.
What HiFi article detailing the issue
Fortunately Panasonic have corrected this issue with a firmware update although I'm not familiar with whether the G15 supports web update. I know the G10 which I have doesn't and you need to contact Panasonic support and ask them to send you the latest firmware on an SD card. I have a copy of the G10 version but I doubt this will be compatible.