I have never heard of someone having problems with the memory card part of the PS2 hardware... everybody and their brother's dog has disc problems, but the memory card slot?

One question, are these memory cards Sony branded? I have heard of people having trouble with the cheap knock offs. If the card has become corrupted it is very possible the PS2 is hanging when it tries to read the card. Either way, I'd suggest buying another Sony brand memory card and see if the problem continues... if it doesn't, try copying over game data until you find the culprit...

...but then it's very possible you've tried all this. In that case... oops.