Maybe a stupid idea.. but is there any chance that your image has a 1/4" white border that you can't see (because it's white)? Have you tried printing any other image?
Not a stupid idea and I thought that may be the problem... but it wasn't that.
I tried a bunch of different 'canned' paper sizes today and it finally worked on the 13x19 (Centered) setting. It supposedly put a .56 margin at the top and bottom, .12 on the sides... and since my image was 10.25 x 15.25 - well within the borders. It printed perfectly. At least I got my job done! Why this didn't work like it has for the last two years I still have no idea.
I would still like to know why this is happening, but Epson couldn't figure it out either so I probably never will. I usually print on roll paper anyway, but for certain smaller jobs, sheets just make more sense.