For the last couple of days my laptop's screen has been showing a 1 pixel vertical stripe, bright green, appearing at random times but always in the same position. Stays there for some minutes, starts blinking, suddenly disappears... over and over again. What would you suggest? Should I start looking for a replacement screen?
I'd say that there are 3 most likely causes.
First could be a bad graphics card, as others have mentioned.
Second could be a bad cable which connects graphics card and the screen.
Third could be damaged screen.
I don't see an easy way to determine which one of the three it is.
It would be best if you could make logs of what you did when the stripe starts showing and when it disappears. Things like temperature or time the computer was on could be relevant in addition to which program is running and how big load it is placing on the graphics card.
Also, you could try gently pushing the screen near the line once it appears. It there's a broken connector in the screen itself, you could diagnose it this way.
As for the cable, you could try shaking the laptop and taking note if anything happens. Unfortunately, that's not a good idea, since it can damage hard disk drive, optical disk drive fans and other moving components. Maybe tapping the lip would do?
If it's graphics card, try running some very demanding games or test programs on it to see if the line shows up.
In the end, you could always try to open the lid and check if the cable is connected properly. This way, you'd eliminate one possiblem problem.