Your TV has a warm-up fault - as the temperature increases a component fails and the picture disappears.
An engineer might trace the culprit using an aerosol freezer spray on a section by section basis - gradually on a smaller area each time until the specific bad component is located.
Electronic freezer spray might be had from Maplin or Amazon for about £10 - alternatively a TV doctor might repair your set.
It is sometimes the case that the component that appears to be the cause is not the cause. If A feeds a signal to B and cooling A makes things better it might be component B in error. The TV doctor will be aware of this possibility.
Good Luck...
January 2017