In reply to ClimberEd: Check how the signal is being split. It should really be done with a powered amplifier/splitter which maintains the signal strength to both TVs rather than splitting the power between them (so each gets <50%). This also avoids any noise being coupled back from their TV into yours.
Digital TV uses a complex encoding scheme to reduce the amount of data being sent - and some programs have more action, so these either have to be compressed more, or the other channels which they share with (since all your normal BBC channels are shared on a single frequency) have to be compressed more. Part of the compression is to only send a full picture every few frames, and between those to only send the bits that change. Hence in a weak signal area, or if there is some noise which corrupts the data, the TV has to wait for the next full frame to appear before it can display a valid picture. It may either blank the screen or freeze the last good picture while it waits for good data.
Denzil