Broad vertical bar appears on TV screen

Question

A broad vertical coloured bar appears near the centre of the screen after about 30 mins. The bar width is about a sixth of the screen. Sometimes it disappears later then will reappear. The colours in the bar change all the time. We have had a TV engineer to try to solve this but even when he took it away to study it he couldn't solve the problem. We wondered if it was due to overheating but we have discounted this.