If the problem goes away it's probably not a bad yoke, but I'd
look at the pincushion correction circuitry.

Nick wrote:
I have a Sony KV28WS2U that has worked fine for approx 9 years.

Lately the picture has started to be intermittently distorted. The picture
will reach all 4 corners but be horisontally narrower. The picture has
almost an "hourglass" shape. If the TV has been on for long the problem goes
away, but as time goes on this takes longer and longer to happen. Last night
we had the hourglass picture all evening.