Where exactly is the yellow line? If it's inside the pillarbox on a 4:3 image that would be the station. If it's along the outer margin of the screen then is could mean your TV has relatively low overscan. Most folks actually consider this a good thing as you see more of the image but it can be a little aggravating if artifacts show up from time to time at the image margin.
Some TV's allow you to adjust overscan and image position.