Problem with a Philips 50PF9630A

[Allen1973]

Hi to all,
My 50 " plasma has recently developed a problem.
After being on about 5 minutes, it will sometimes blackout the lower half of the display. This will last 10 to 15 minutes, and then the display will return to normal.
Has anyone else seen or had this problem?
My other thought is how long before the whole display blacks out?
It does this whatever the video source.

Thanks to all in advance,
Allen