PS2 Framerate slowdowns

[nolifegamer92]

I was playing my ps2, a slim schp-75001, and i noticed in parts of sly 2 band of thieves, there were slowdowns in the framerate, and the game was somewhat choppy. sly 3 to some extent has this problem too, but when i play games such as socom 2 and 3, and 007 nightfire, the framerate stays constant. I tried cleaning the disk, didn't help. before i had no slowdowns, but now it's just constant and annoying. i checked the vents and it is not overheating. what could be the problem?