Getting a weird screen flash-out on my Xbox 360

[starcraft]

Hi all,

I switched on my Xbox 360 (haven't in a while, I haven't had time to game much lately) and downloaded the latest update.

Now, whilst the Xbox 360 seems to work fine, the screen constantly flashes in and out.  As though the Xbox 360 is only intermittently sending a feed to the television.

I have checked the cords are plugged in, but to no avail.

Thoughts?  Help please