Seems to be a problem that bounces around the country... Somebody will reset something soon and all will be well for a while. I've seen it on Fox Pittsburgh and Wash DC locals from time to time. Others have posted about it too.
I didn't notice it so much on WFLD but then the way the Bears played yesterday I may have just mistaken it for Bears fans groaning!
I did notice it on WBBM (also Chicago) earlier the day, though. I tried resetting my receiver and it didn't make any difference.
This seems to happen fairly regularly on athletic events - I suspect it might be related to the compression algorithms used and the fact that fast action sporting events don't compress nearly as well as other programming (I am no expert on compression so this is pure speculation).
Been happening for several months on my local FOX channel in Flint, MI WSMH. I would use channel 2 from Detroit but ever since the digital switchover they were one of the few that stayed VHF and I can't reliably keep it tuned in.