Hi is this a plasma TV or LED TV?If you could advise the model of the TV it would allow me to advise you further.If the TV is LED its probably fairly cheap to repair, in most cases its just a printer circuit board which needs replacing, which is a 10 minute job. If its a plasma tv there are a few more things which could cause the issue.
It's a plasma flat screen - about 6/7 years old...one of the first models they brought out in white with the curved bottom....would give you the model number but it's wall mounted can't easily take it down and don't have the paperwork for it anymore sorry
Ok, based on its age and symptoms I would suggest that its going to be uneconomical to repair, it sounds like a plasma fault rather than a circuit board.
The regulator which forms part of the screen controlling the plasma sounds like its faulty.