mitsibushi 1080 tv series issues

Hey there, new to the site...  Having an issue with my tv and need some help bad.   Not real tech savvy, but I believe it is a rear projection about 56" tv (manual is missing at this point).  The TV is about 5 years old max and several times a day the picture will suddenly 'expand' and hten shrink back to normal, then 'expand' again.  It does this for about a minue or two and then just shuts off.  Any thoughts or suggestions on how (if?) to repair this issue???