My tv sony kv-pg21n70 has horizontal white thin line about 2 inches and blue ,green ,red thin line under it

Sound like your Y-Sustain board has one foot in the grave. The Y upper buffer and Y lower buffer boards go when the Y Sus Goes too...

Nothing you can do about that...get used to it or change the TV.
It's a defect in the panel that can't be reliably fixed.

Vertical retrace lines is the problem. Thin white lines runs across the entire screen, at about an inch apart to each other. Check its video output supply voltage. Reduce the screen voltage to a little bit down and check. If you still wish to know more about this fault, check this site. Pull up older posts there.

Measure B+ on the set , If normal check G2 voltage


G2 adjustment is on the Flyback Transformer

I would look for a Vertical circuit failure. Most likely is the vertical output IC and or open resistor off the hv transformer. Definitely look for any loose solder connections around the Vertical output IC.
Hope this helps,
Skyassoc

the problem there is a loose connection on the board,you said that you smacked the right side of the tv and it goes normal.there are parts that should be resolder on the board in order to back into normal operation.

This is for a different model, but I had thin horizontal lines in red and had to adjust the tube for red. You may have a similar adjustment on your set.

http://www.somelifeblog.com/2007/06/sony-projection-tv-red-line-repair.html

if the set has a previous vertical line problem and a white horizental line only comes in the cross of the screen and you turned the tv on for a time while the problem is solved that mean you have make a burnt line to the screen , but if you are the tv owner and nothing of that happens so you have a problem in faulty dried capaciter in the vertical live part search for 33uf 160 v or 22u 160 v and alter regards please don't forget to rate thx

This is possibly a sign that there is a fault condition. Short circuit line output transistor

TWO Blinks---B+ over current protection (OCP) , unit goes to the standby mode then displays the 2 blink fault. Could be a short in the power supply of any of the circuits.