Zenith SR2768S small thin line

Hello, i just recently got this Zenith for free. It appears to be a 27" screen (from visual inspection, and looking at the model number). I have done some breif looking on this forum and am not really sure where to start. The screen will work fine for a few minutes, and then will start to shrink down, top and bottom will shrink into a smaller picture or into one line in the middle of the screen. What should i start with, and where should i start?