Your TV set has a warm-up fault in the section of the electronics that processes the subtitle feature.
A trip to a TV doctor might be able to find and cure the problem - the technician alternatively cooling and then warming sections of the circuitry to eliminate and induce the fault condition to hunt down the offending component.
This would likely be a costly repair. Although the repair engineer might be able to zero-in on the fault quickly.
Your other alternative is a new set.
There is very probably nothing you can do to fix it yourself.
Good Luck...
June 2016