I have a Sony KP-51ws500 that has the blue and red horizontal lines going across the black screen... has anyone else ever seen this,,, the image is ok accept when you have a dark screen

The Blue CRT ( Picture Tube ) has a short. You may be able to do a temp fix by turning down the Blue CRT drive level ( Located behind the front speaker grille )untill the lines disappear. But its only a temp fix. The Blue CRT will most likely need replacement.

Red and Blue Tube screen drive controls needs to be turned down some, tubes are over driving causeing drive lines.

crts are bad

Off hand I would con dim the Red and Blue CRT"s. If you were to replace two of them I would also do the Green CRT as well. The cost of just doing two of them is way too expensive in my opinion and you would be better served to replace the set instead of investing the amount of money needed in a 6-8 yr. old set.
Hope this helps,
Skyassoc

had tv that did the same thing after three years but they were green cost me 300 bucks to get it fixed and the guy said every 3 to 5 expect it

You need to turn the voltage on the red tube down slightly, also you would need to adjust focus to compensate for the lower voltage to the red tube. On the left and right bottom side of speaker cover are two round covers, take them out and there are 2 1/4" hex screws take them out carefully take cover off, there is another service cover same hex screws remove. You will see the focus block on the right side, tubes are labled on block. 2 knobs for each tube. Make voltage adjustments while tv is on a black screen, then for focus, turn color all the way off in the menu, and adjust focus until it looks right in black and white. These adjustments should only be very small; if alot of adjustment is needed, you may have other issues.

Sounds like a convergence problem:
http://www.fixya.com/support/t257424-colors_out_alignment_curved_lines

Hello,
You need to adjust (actually bit decrease) G2 volts (calls Screen volts) of Blue gun.
If you are familiar with big TV repair and have electronics knowledge then let me know to guide you further.
thank you.

does it have blue video image?

A "Gray Scale" adjustment may solve you problem. If one of the picture tubes is set too high it will cause these RETRACE lines. Then too a weak picture tube might cause this. If the horizontal lines are one color: red, green,or blue then whichever color that is shown would tell you which tube is set too high. Your set has 3 screen controls that control the brightness of each tube. One may need to be turned down a little. Stargazer