samsung LE40A656A1F - screen goes blank?

hi

i've been having issues with my samsung LE40A656A1F LCD screen goes blank but still has sound.
this occurs when i'm switching between different sources.
i've found that turning the TV off then on again solves this issue for a short term but then it reappears again when i'm switching sources.

I have 2 hdmi devices connected, cable TV box and a bluray player. i've tried using different hdmi cables and using different hdmi slots on the TV but the issue remains, i've also tried reinstalling the firmware but again still has the same issue.

hope that someone can help me solve this issue, thanks in advance for any help.