MIDI hanging notes problem

Whether I use my KN7000 into my Apple MacBook sequencer via USB directly from the keyboard or via an M-Audio USB to MIDI interface, I keep getting hanging notes which are only cleared when I play the same note again.
The keyboard plays perfectly correctly using the internal sounds. It is only over MIDI that it exhibits this behaviour.
It is like the keys sometimes (never the same keys) do not send a note off message. Replaying the note can overcome this as the note then does send a note off message.
Anyone else experienced this problem/know a solution, please?
Thanks, A