picture distorted philips 42pf9946/12

[pakman]

i have a philips 42pf9946/12 plasma tv.
after watching the tv for aprox 1 hour the picture becomes unstable until the screen appears scrambled (similar to when you used to watch an encoded sky channel on the old analog with no card)then it switches off. if you switch back on again all is fine for 10 mins then same again.
any thoughts?