NTSC = 60 Hz == 60 fps processed, 30 out
PAL = 50 Hz == 50 fps processed, 25 out
Often plasma screens are susceptible for blurry images. Blurry images result from to low framerates. Normally to low framerates are circumvent by interpolating to images to get an picture between those. So that's what a Plasma TV (maybe LCD as well) do. The difference between interpolating from 25 respectively 30 fps may result in a difference, which is noticable by human. Newer Plasma screens should still be able to avoid bluring by software...
However as a rule of thumb: the bigger your screen is, the more vulnerable it is for blurring.
So if it's real "blur", that might be your answer.
I hope I explained it the right way.