Something must have changed for the problems to start. Even if it was at the broadcaster's end. My philosophy is that it pays to make sure we cover all bases with a problem like this... even the ones that seem like they wouldn't need to be checked.It's not the processor folks, can't be. Everything worked just fine a little while ago, HD the lot. Live feed throuh HD was fine. Logically therefore it just can't be the processor load.
Even when the channel it is MPEG2, it would stress the ATI 2400 too much if it is 1080i. Try disabling the deinterlace in Catalyst settings completely (sorry, haven't owned ATI have for few years so I cannot tell the exact place where to check).