Sony BDP-S390: screen goes black but sound continues

[Kookoo]

I've just bought a Sony S390 blu ray player which is hooked up to my Panasonic (TX-37LZD85) 1080p TV via HDMI. However, within a few minutes of playing something (a disc, a USB or streaming content) the TV screen goes blank for a couple of seconds. The audio continues during this time, but not the picture. The onscreen "input" label flashes on in the corner of my TV screen (not the full label, just "WIDE"), and then a couple of seconds later, the picture returns.

All settings are correct and firmware for both TV and player are up to date. I've also tried different HDMI cables, but with the same result.

Can anybody tell me what is causing this? Even though it only lasts a couple of seconds, it is extremely annoying because it happens every time I use the player.