I promise not to bore you with loads of 'techie theory' about signal-splitting and standing waves but both adding a new device (TV, digibox, DVD recorder) and subtracting an existing one from a system can adversely affect the signal strength so, yes, it's possible that the new TV may be a factor.
However, the previous reply was completely correct in diagnosing the prime cause of your problem as insufficient signal strength from your aerial. In a technically perfect world, every TV aerial would be perfectly matched to the signal of the incoming frequency of the channel you want to watch. The problem is that you'd need a different aerial for every single channel! So all TV aerials can only offer a 'compromise solution', which means that they're better matched to some channels than to others. (I've also seen a post, here on AB, which said that the ITV and C4 channels use a different type of encryption to others, requiring a slightly stronger signal strength. That would certainly explain why it's usually those channels which people have difficulty receiving).
First, step outside a check that your TV aerial is still pointing in the right direction. (A battering from our wonderful British weather can often lead to problems).
Assuming that your aerial looks OK, try adding a simple signal booster. (From larger branches of Tesco, Asda, etc, or from Maplin Electronics). That might well be enough to fix the problem. (You won't really know until we get some heavy rain or snow, when signal strengths are at their weakest).
If a booster doesn't work, upgrade your aerial system by replacing both the aerial and (just as important) the cable. This 43-element aerial has been widely praised by AB contributors:
http://www.maplin.co.uk/module.aspx?moduleno=2 2523
Chris