first...is it with all your UMD's?or just one..if its just one..check if any dust/small scratches are on the disk..
if its all of them..it may be your eye that reads the UMD which is broken..which..indeed..is bad..if this is the case..then go and get it fixed..send it to sony or something
if its(again) with only one UMD disc..it may need a required update....
PS

ost ur update on this news please..thnx
