D.J.KRIME:
This could simply be that the set is re-adjusting itself to the resolution of the input that you are selecting EG if you have switched from a SD 576 input to a native 1080 one (assuming your TV is Full HD) there will be a brief period where your screen will go blank whilst it adjusts to the new input resolution.
This also happens if you have SkyHD and your box set to AUTOMATIC where the box outputs a channel at its native resolution leaving the screen to do any further scaling, so when you go from say BBC1 to BBC HD there will be this slight pause in the picture being displayed as the to adjusts to the new picture resolution.
Yes this is true. Further to this point I find that the implementation of the AUTOMATIC resolution setting on the new EPG software is rather annoying. I always used the AUTO setting because the upscaler inside the SKY HD box for SD channels is laughable and when I let my Denon receiver do the upscaling SD channels looked loads better (it also locked onto the new resolutions in less than half the time it would take my panasonic telly to), but with the new HD EPG if you set the resolution to Auto it makes the EGP text very pixelated and unreadable as if it's being rendered in SD at all times. Also found that in Auto mode pulling up Mini TV viewer between HD and SD channels made the box crash a few times.