Hello Andrew,
This is something i have never had with my Panasonic plasma and as far as i am aware it's not an issue with plasma tech. It could be an issue with settings such as having some porocesssing going on in the background which may be causing the issue, are you using the motion processing on the tv for example? Also any MPEG or picture noise settings can cause these sort of issues so go into your menus and make sure that all picture settings that can be turned off are indeed turned off completely, anything that is a picture processing tech is better turned off as a rule.
If this doesn't solve the issue then it's contact Samsung and/or your retailer to find out if this is normal for this particular tv i'm afraid or live with it if it's possible for you to do so.
Hope that helps, best regards