imread issues when loading large TIFF's

I have large TIFF image files that I am loading (i.e. 80k by 60k by 3 arrays) and when I call portions of them using 'PixelRegion', there seems to be inconsistency when I run it every time. In other words, sometimes I run a y range of 1:1500 and x range of 1:1500 and the program outputs the bottom right of the image, whereas it should output the top left. Sometimes when I do this repeatedly, other portions of the image are outputted. I am not redefining any variables and am simply playing with this in the command line. Has anyone ever had this issue?