I bought a brand-new Intec 1x memory card almost a year ago. It worked for awhile but started to do the same thing as your card: you can't save anything on it. GRRR. I erased all the memory on the card and it still says Memory Card Full, Change Slot. It does this on both my PS1 and my new PS2 slim model. It pissed me off because it was new when I bought it. Now I only buy used cards.
Also, I noticed that both my Nyko memory cards will not work on PS2, but they'll still work on the old system.