i had a 2000xl for 6 years before switching to the 2500. this "the MPC2000 never gave me this problem" is ridiculous, so drop it. both machines have the same polyphony.
read up in your manual about program polyphony and you'll be able to fix your problem. the mpc2500 defaults programs to mono, meaning every pad cuts off the sound from every other pad within the program. you have to turn that off if you want different pads to play at the same time.
this question is asked every few days. in fact, i'm pretty sure i answered basically this same question last week.