TV (KDL-40NX800) shuts off when combined with BD surround system (BDV-L600)

[tarsius]

Hello all

I have a Sony Bravia KDL-40NX800 television since december 2010.
This week I bought a Blueray Home Theatre system Sony BDV-L600.
the connections are as follows:
Digital cable Set-top box (BelgianTelenet) --HDMI--> HDMI1 (in) of Bluray system
Bluray system HDMI (out/ARC) --HDMI--> Bravia HDMI1 (ARC)

When playing DVD or BD disks, the Bravia TV switches itself off and on again at irregular intervals (seconds up to minutes).  This also happens when watching broadcasts or recordings from the STB, however less frequently (10-20min).
This never happened before I attached the Bluray system.
Watching TV has become a nightmare!

Bravia firmware: PKG4.107EUH-0108 (latest)
Bluray firmware: M07R0427 (latest)

No eco settings are activated.
When the TV switches off/on, the sound remains so the broadcast or disk keeps playing.

I would expect Sony hardware to be compatible with other Sony hardware...
Can someone please advise? for now i switch back to my old setup, skipping the BD player/surround sound system

thanks