was googling for help on my issue and found link to 'get expert help on playstation network', so here goes.

I bought a brand new PS2 slim and new Guitar Hero 2 two days ago. Finished all the songs on easy mode with no technical problems. the next day when i started on medium mode, the PS2 started taking over a minute to load each song and while playing the song the game would freeze up for a second or two several times during the song. now I can't play any songs with out it freezing up all the time while playing.

Is it the PS2 or the Game? Should I return both and try again?

Check your disc for any scratches- if you are not careful with the game discs they can scratch up easily. If the disc is free of defects try another game in the system, just to make sure it is working as it should.