TV: 40PFL9606

Every couple of hours (sometimes 1 hour, sometimes 2 hours) the:
TV screen: black
+4 seconds: Ambilight: black
+4 seconds: Ambilight comes back
+3 seconds: TV screen comes back
(timings are indications)

This happens regardless of the source (Humax 5200 on HDMI2, Mac Mini on HDMI1)
I have exchanged HDMI cables without luck.

Any ideas? Known issue?