My Dad bought a KV-36HS20 about 4-5 years ago from Audio King. He purchased cheap from them because it was returned(I know, big misktake). Ever since he purchased the TV, there would be a pop or crack sound followed by a green horizontal line flashing across the screen. The flash occurs more frequently the longer the TV is on. It disappears after a second.

CRT dude,either intermittent short from cathode to heater or cathode to G1.
50-50% if rejuvinator can removed the short.

i agree it's the pix tube. try laying tv on it's front and tap lightly on the neck of tube. maybye it will remove shorted debree.

Update: Today I plugged it directly into the wall outlet instead of a power strip and now the lines are White. I have never seen the white lines before. Also, it is flashing/crackling very often now.

it's the tube. try rejuvinating it with it laying on front.