Picture stretched or "squeezed" inward from sides - black section on side.

My parents gave this TV to my son. They bought another one at the suggestion of the repair shop person who didn't even look at it. Basically the picture looks like somebody grabbed the middle of each side edge and pulled them in toward the center leaving a convex black vertical bar on either side and compressing the middle portion of the remaining picture. The TV will then eventually shut itself off. I'd especially like to know if this is a picture tube problem. The picture tube is still under warranty but my folks didn't take it in to the shop because they were told that if it turned out to be a non-picture-tube problem, they'd have to pay for the diagnosis AND it would be more cost effective to buy a new TV then to repair. Incidentally, my son has used the tried-and-true repair method of whacking the side of the TV chasis and this will sometimes correct the problem until the TV is shut off manually and then the problem will usually reoccur when the set is turned back on. Often, when he smacks it, the set will just turn itself off. I opened the chasis to check for any obvious problems and attempted wiggling various wire connections and circuit boards and nothing presented itself as the obvious source of the problem.