Panasonic Viera - screen going black?

Hi i am having problems with my Panasonic Viera TV. i have had it for 2 years with no problems but recently it has started with this problem.
I am watching the Tv as normal through my cable HDMI linked to a V+ box and every so often the screnn will just go black, only for a few seconds and then come back again. This can get a bit annoying if it happens at a crucial moment in a programme.
Thanks for your help