Just a couple of ideas - can you, or have you tried a different quarter inch foot? even a fraction of a difference can be the "fix". Also - don't know what kind of foot you're using, but if you use the kind with the stop on the side, is there a chance that is bent and needs to be fixed or replaced? If your needle position can be changed, have you tried changing the position of the needle with the foot to make the correction? Is it possible that you're expecting a leve of perfection that is not needed - I admit that I'm not as exacting as many quilters about the size of the seam - if I'm not doing points on a star, no one but me can tell if I'm not exactly where the books say I should be. My mantra is "consistency not necessarily perfection." good luck in finding your solution...and remember the galloping horse theory of piecing...if they can't see it riding by on a galloping horse, it's not a problem.