bugi , when you say you checked the memory, did you format it with MP3 Utilities Disk Tool or you opened and verified the chip number against the manufacturer?
In some cases, the chip number might be stamped wrongly, a 256 or 512MB might be stamped 1GB.
I'll suggest a format if you haven't yet done so, the guide is
Here.
It could be that some parts of your memory chip might be corrupted or have bad sectors.