Slightly noisy picture despite strong signal source and dark shadow

[chappers]

Has anyone got generic advice (although the LCD TV in question is a Samsung 710MP with MH17AS chassis) on why, despite a strong signal being present (analogue in this case from a Sky satellite receiver) of around 65-70dB, a set might display a very slightly noisy picture, akin to signal being slightly too low?

Another effect I have on this set, only on analogue RF, and NOT on SCART, etc., is that areas of dark shadow, such as the edge of a hand or face, are appearing as thick jet-black areas instead of graduated tones. With a face, it looks like a big paint brush has been used to paint a jet black thick line down the edge of the face. It is not affected by brightness, contrast changes, and is only apparent on RF input.

Thanks