Hi Mark,
I have had the same problem at some locations - including the two shots I posted above. I have attributed it to what I think (speculation really) are high pressure sodium lamps that cycle with the line current. Not only does the luminance vary, but apparently the color temp does too. I do a click white balance for each shot I want to keep (using the ball or numbers on jerseys as white reference), but some colors are still off across the frame - is there such a thing as a color temp gradient to apply in post jrw? I haven't noticed this effect when the bulbs are clearly simple fluorescents.