I had similar problem for the first time today. one key (c4) triggered the sound propperly, but it didn't transmitt duration when holding the key down. it sounded like a turned down sustain on the amp envelope, but i had sustain fully cv and it worked properly on all other keys. and just like sacred synthesis said, the problem went away within a few minutes of playing. might be according to dust on the inside as i have my p8 for 3 years now.