Most likely, it's because you've moved the TV's position relative to either a local magnetic field or the earth own magnetic field.
There's a circuit built into the TV to clear it automatically. Use the TV normally and it may well fade and disappear when the circuit activates during it's power up. (Don't switch the TV on/off repeatedly to try to speed up the process, as it may cause other problems and it won't work anyway.)
If there are any loudspeakers near the TV, move them further away. Large radiators, power cables, metal frame buildings can be a cause of local magnetism. Try moving it away from any very close wall.
If the impurity persists, a TV engineer can degauss the TV and it's surrrounding area.
Cobweb, November 2005
