Actually good old CRT:s handle low def material better then a high def plasma (or LCD for that matter). The problem with sports on analog broadcast is that they are usally sent in letterbox. Letterbox in analog broadcast means even lower resolution then 4:3 in analog. This is because that the letterbox has about 25% lower resolution.
With digital broadcast however the picture is 16:9 to begin with and you will have the full standard def resolution even in widescreen. When your using an plasma or LCD-tv you will benefit A LOT from digital TV.
One thing you should really try tough is to shut of all picture-improving settings (such as noise reduction, mpeg nr, 3d nr etc). When the signal/ and or resolution is really poor theese "improvements" often hurt the picture more then they improve it. For eample: noise reduction will always remove details as well as noise, which will hurt the quality of the picture.
Shutting of all picture improving settings improves the quality of analog letterbox broadcasts a bit for my plasma (LG px3rva which is however low def). Also try to lower contrast and brightness and most importantly lower the amount of color - they are often set to high with will simply blur. Also try a color temperature that is less red (e.g switch from warm to cold). Many people choose the "warm setting" because they think it looks nicer but it will exaggerate the red parts of the picture which could be the source of your problem.