I have a 4x5 sinar f1; I shoot primarily portra 160 VC color film (ready load) with a polaroid back. The back always works well but sometimes I get a thick band of off-coloration when I scan my film. It is about an inch wide and stripes across the 4inch width. It appears as a yellowish fog that I am usually able to digitally remove, but I want to figure out what could be causing it. Does anyone have a clue? Might it be in the processing?