I have a Ps2 slim version and some games such as Baldurs Gate Dark aliance II, Gauntlet Seven sorrows, Castlevania lament of innocence, Dark Cloud and some more all have the same problem. The games all run, but the screen start going up to down continuously making these games unplayable.
I don't know if any of you already had VHS movies in the past, but I remember that the same problem occured with some VHS and the only thing you had to do is play a little bit with the "tracking" wheel to adjust the screen. That's what I mean by "going up to down". The difference with the ps2 is that it's only with some games (not all of them) and it's not a VHS...
Try another tv, unlikely thats the cause. Do you play those certain games that cause the problem at a different frequency, i.e pal-60hz, rather than 50hz?
If it's pal version, does it mean it have a language selection screen at the beginning of the game? Because all the games I mentioned had one...
So, maybe the problem is pal?
I tried all the games on another tv and the problem turn out to be more playable...
Instead of going up to down, the tv screen is too big. I can't see the bottom of the screen (about 15 cm). I tried to play with the tv setup to "squeeze" the top and bottom screen together, but I didn't find any options that could let me do that.
Well there was a setting set to "color" and I changed it to "pal 50" (that's what you said I think about the frequency). At first it didn't work, so I went back in the config screen of the modbo chip (triangle+square when you boot up the console if I remember correctly) and I changed the "y axis" option to "auto adjust".
Everything work with these settings (even the games that were working still work with these new settings).
There are 2 programs you can get off any torrent site called PAL2NTSC or NTSC2PAL. I live in Canada & have an old TV so any PAL game I download I have to use PAL2NTSC or my screen will always move like a slot machine, some PAL games already have a selection between 50 Hz or 60 Hz built in. The only problem with this program is in some games the screen will expand so you lose a bit at the top & bottom of the screen. The best fix is to save up & buy a new TV as newer TV's accommodate the different Hz between PAL & NTSC; I'm still saving money for my dream TV.