Are you sure its the PS3 as ive had this problem with old tv
Its like imagine a role of old school camera film. Well its my screen rolling downwards looking exactly like that with a black line separating each screen as it scrolls downwards.
I had this problem with a scart outlet years ago when i had the ps2 again though it was a faulty TV, its like a flickering screen, like an out of sync projector or something hard to explain but I know exactly what your talking about, its probably faulty from the inside, maybe lying loose if you footer about with it alot
Id go with your Nans she probly wont even notice the difference lol
Well good luck with it, hope you get it sorted. I know I couldnt go back to scart after playing HD
my next thing was going to betry a different tv / hdmi cable but now you have them that should sort it. if not go to settings then display settings and make sure you have the correct settings hertz and everything.