DM800HD strangethings happening. Please help!!!!!!!!!!!!!

Hi I have a strange problem. Installed a dreambox800HD about 2 months ago all worked ok. Untill a week ago when i turned it on and booted screen displayed showed up ok but no display on the tv after removing cables ect.. I eventually traced problem down to when the sat cable was fitted the tv display would go off and when removed would come back on. Thought this was the problem initially but then I reconnected my *** HD box and when connected getting picture as normal and checked signal strengh again is excellent. I have now borrowed a nother dreambox from a friend and am getting the same problem so its not the dreambox. To be honest I am not sure what to do next. Should i try a different output from the *** dish as I have a octal LNB but if i do why am i Icould understand if i was geeting no signal but works fine with the ***box and was working with the dreambox. As i had two sat cables for the *** box tried both. Both cables were connected to the dreambox which is a in and out would this affect it. I have tried the dream box in another room and is working ok PLEAS help somebody!!!!!!!!!!!!!!!

hi have done that but still nothing on two dreamboxes yet is fine on either sat cable or both connected to a *** HD box and both signals are excelent strength ect.. and get a good picture but as soon as sat cable is connected to dreambox the video image (HD) goes off and only comes back on when i take the sat cable back out again????