Hi Pavel
Funny you should mention this as I sent off for a cheap optical cable to try and - it seems to work fine I have not used it for long enough to be sure but the music isn't interrupted when I turn other equipment on or off. I have not used an optical connection before and, on this cable at least, the plugs poor and don't fit securely.
From what I understand the coax connection is the better one to use
Anyway, it seems the coax connection is at fault.
Anthony