MITSUBISHI BLACK DIAMOND TV 28"?

I HAVE A 28INCH MITSUBISHI BLACK DIAMOND TV,ITS ABOUT 7 YEARS OLD AND HAS GIVEN GREAT SERVICE UNTIL THAT IS .RECENTLY THE PICTURE WENT NARROW ON THE SCREEN WITH THE PICTURE IN THE CENTRE AND BOTH SIDES WHER IT HAS BEEN COMPRESSED ARE BLACK,GIVING A "SQUASHED UP" APPEARANCE.IT SOMETIMES FLICKS BACK TO FULL SCREEN PICTURE(AS NORMAL) BUT THIS HAPPENS LESS FREQUENTLY THA IT USED TO,MOST TIMES THE PICTURE IS IN ITS NARROWED"STRETCHED FROM TOP TO BOTTOM" FORMAT.I DROPPED IT INTO A TV REPAIR SHOP AND THEY FIXED IT AND IT WORKED FOR A WHILE BUT NOW ITS BACK AGAIN DOING THE SAME THING.DOES ANYONE KNOW IF THIS CAN BE FIXED PROPERLY,WHAT IT IS CAUSING IT? IS IT THE TUBE IS DETERIORATED.YOU CAN BE WATCHING TV WITH NORMAL PICTURE ONE MINUTE THAN THE NEXT IT JUST FLICKS TO THIS NARROWED PICTURE.CNA ANYONE OUT THER HEPL ME WITH THIS.
THANKS ALOT,
LJ