guys, need your opinion on this

i bought my PS2 slim 3 years back from a forumer here, second hand unit. The PS2 been running fine until last week, it had difficulty reading some games. It's not that the PS2 can't read them at all, the game runs fine but it will get stuck at specific parts of the game and loads like forever. For example, in GOW2, the game gets stuck after the barbarian boss fight, in GOW, after saving the Oracle, and in MGS3, after the Shagohod fight. All these games are original NTSC U/C discs. My PS2 model is SCPH-75004, PAL region. Anyone knows how I can fix this problem? Thanks in advance.