What is causing the bad picture on the internal display only

Xandu diagnosed the problem as a defective screen (not a defective Nvidia chip 86000 GT), so I replaced the screen and as this did not fix the problem I also replaced the display connector and the LED driver board without success it appeared at first. However, after starting up as single user the picture started to improve but also went bad again; low resolution, every other vertical line seems missing, low contrast. After repeating the single user startup several times it stays good for about a week now. What is happening? Were the replacements necessary?