Not sure, I gave it to a repair guy and he said the graphics IC (is there such a thing?) is damaged. He swapped the damaged one with a working one from another used PS1 for a minimal fee. It fixed the problem, but only temporarily, a few weeks later the dreadful lines were back. I got it repaired a total of 4 times I think! And then just shelved the console.
P.S. I forgot, the problem doesn't just occur in-game, its everywhere. Even during the console start-up when the Sony PS1 logo displays, it has black lines crossing all over it.