Wow, thanks for all these answers! :O
Thanks 73s de Phreakout, you're awesome and very helpful here :-*
Well, it was not kind of waving after reflexion but more like showing glitches, artefacts. But slapping it a little bit revives it so I guess it would be cold solder joints. And nope, no pattern at all.
It is wide, well I'll check out everything you told me, going to pull appart the beast today.
Might be the RAM or the HDD, again I'll check this out today.
Thanks 73s de Phreakout, you're awesome and very helpful here :-*