NEC Television five years old PF-68T31 has intermittant fault white horizontal line across screen

Backlight CCFL lamps fault. Contact any service technician. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" will make surf easy. Surf the site with patience.
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Horizontal output section has the fault. Check it. Replace horizontal output transistor.Try this link. http://electronicshelponline.blogspot.com/ There are more than two thousand and five hundred posts. All are related to television and other domestic electronic equipment service help.

Unplug it,then wait 10 minutes,then plug it back in.

If you wish to get some details; visit the site linked here. Pull up older posts.
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Line faults & possible causes to LCD & Plasma screen are given with illustrations. You can get an idea about the fault to an extent. Faults to all LCD & Plasma screen are similar, irrespective of its brand and screen size.

Replace all the electrolytic capacitors in the vertical output section circuit. There will be 5 or 6 of them. Replace the at one time. The fault you see is technically called " vertical retrace lies".

I would test the cable, use it on a different monitor or try a different cable. That is the only other thing to check, otherwise it definitely is the monitor.

check the supply at horizontal driver trans former ,if voltage is correct at ho/driver trasistor ...then check allthe fusible resistances from power supply,,,if tv is taking h/oscillation for awhile ,,give more screen control in lot,,, & see for a horizontal line ,,,before tv becomes off,,, if so vertical ic problem...

replace the focus block or try to adjust the screen control on the focus block for red only. this will affect the white balance of the set so adjust it carefully. the focus block is located on the front inside the set. remove the speaker cover then the 4 screws on the front plate. the focus block is 3" x 2" black box with 6 adjustment posts three for focus and three for screen. adjust only the affected screen (R).

Replace it, it is going bad. I could tell you what would cause this such as bad driver IC etc. but bottom line is its five years old, you got your moneys worth out of it so bite the bullet and get a new one.

Sounds like the flyback transformer is bad. couple of 100 dollars if it is that