If the power goes out completely on the keyboard, then I'd suggest what Gary wrote. Check the power supply. If the wal-wart itself is good, it could be the connection within the keyboard.

If the power doesn't go out and the midi file just stops, then it could be a bad midi or dirty drive.

I've had a few midi's lock up on my boards--there are some crappy midi's out there Hopefully the problem is only minor.

Squeak