Your frame rates should be a lot smoother than it sounds like they are, stodge. It could be the DVD drive in your PS2 is malfunctioning a bit, just having trouble reading the disc or maybe even an overheating problem.
I would suggest checking all possibilities- check the disc for dirt/ scratches and clean it if need be, clean the PS2 lens using a lens cleaning kit (available at most stores selling electronics) or even try taking the vacuum and use it on the vents of the PS2 to get any dust that may be inside preventing the warm air from getting out.