Troubleshooting MPC 4000

My problem I'm having is on the playback of my composed music. When playing back my music the MPC 4000 studders or Hiccups which leads to the sound playback to sound out of sync. I have tried a few things and I'm not really sure where the problem lies. OH, It's important to know that on both my MPC and my Motif ES 6 an error reading, "MIDI BUFFER FULL" has happend quite frequently. Not all the time but sometimes.

Can anyone help me?

Thanks,

J